General Motors (NYSE:GM) said that it sold 3,483 Chevrolet Bolt EVs in the U.S. in the second quarter. That's a decline of 22.6% from the same period last year.

More to the point, that wasn't enough for the little Chevy to retain its bragging rights as the best-selling battery-electric vehicle in the U.S. not made by Tesla (NASDAQ:TSLA). The Bolt was handily outsold in the U.S. in the second quarter by Nissan's (OTC:NSANY) new-and-much-improved Leaf.

After just a year and a half on the market, has the Bolt run its course? Is it doomed to fade as Tesla finally ramps up production of its compact Model 3 sedan?

Not quite. Let's take a closer look at the Bolt's sales history.

A red Chevrolet Bolt EV, an upright small hatchback, with a kayak on its roof, driving near a lake.

U.S. sales of the Chevrolet Bolt EV are down sharply from last fall's levels. But the reason isn't what you might think. Image source: General Motors.

A lingering question: Why did Bolt sales collapse in January?

The Bolt's U.S. sales have followed a curious pattern. They ramped up sharply through 2017, as you'd expect -- then dropped dramatically from December to January. As you can see, they've stayed near that much-lower level since.

Chart shows U.S. Bolt sales by month. Sales rose steadily through 2017, fell sharply in January 2018, and haven't yet recovered.

Data sources: General Motors, InsideEVs. Monthly figures for April, May, and June 2018 are InsideEVs estimates, as GM now reports U.S. sales quarterly rather than monthly. Added together, the estimates match the second-quarter total reported by GM on July 2.

Is this just about the ramped-up competition from Nissan and Tesla? That's the easy answer, especially if you're a Tesla fan. But it's probably not the right answer, or at least not a complete answer.

The surprising factor that is probably limiting U.S. Bolt sales

With U.S. sales down, you'd expect Bolts to be piling up at dealers. But that's not happening. On the contrary: Supplies have been somewhat tight for months.

Managing the supply of vehicles on dealers' lots (called "inventory") is a tricky science. Dealers like to have enough cars on lots to offer buyers a selection of colors and options. But if they have too many, they may have to discount them to sell them all.

Dealer inventories of vehicles are expressed in terms of "days' supply." For a model like the Bolt, somewhere between 65 and 75 days' worth is probably ideal -- or just over the "60" line in the chart below.

The chart shows a high level of inventory at the beginning of the Bolt's national rollout in the summer of 2017, that dwindled sharply as the year went on, to a very low level in January. Inventory has since been replenished and is holding steady between 60 and 70 days' worth.

Data source: Automotive News. Chart shows days' supply of Chevrolet Bolts at U.S. dealers as of the first day of each month from July 2017 through June 2018. 

GM had lots of Bolts in inventory last July, as the car's national rollout was getting underway. But since then, if anything, supplies have been tight -- very tight in late 2017 and early 2018.

The thing to know is that all Bolts (and the similar Ampera-e version) are made in a single factory, in Orion Township, Michigan. It's believed that factory is set up to make around 30,000 Bolts a year -- or roughly 2,500 a month, on average. Given that U.S. sales of the Bolt broke 2,500 in the last four months of 2017, the low inventories at year-end aren't really a surprise.

But there's more to the story now.

It's not just about the United States anymore

The press release that accompanied GM's second-quarter sales results included an interesting statement from U.S. sales chief Kurt McNeil: GM plans to increase production of the Bolt starting in the fourth quarter, because global demand has exceeded supply:

"Demand for the Chevrolet Bolt EV, especially in the United States, Canada, and South Korea, has outstripped production [emphasis added]. The extra production coming on line should be enough to help us keep growing global Bolt EV sales, rebuild our U.S. dealer inventory, and bring us another step closer to our vision of a world with zero emissions." 

Translation: While U.S. inventories of the Bolt have recovered to a reasonable level, global supplies of the Bolt remain tight.

GM hasn't talked a lot about it before this week, but the company has been exporting Bolts to other markets, notably Canada and South Korea, as McNeil said. GM is also building the slightly modified version of the Bolt,  the Ampera-e, for its former Opel and Vauxhall brands in Europe. (GM sold Opel and Vauxhall to Peugeot last year.)

When you consider that the Bolts that GM sold in Korea in May and June were probably made in the U.S. a few months earlier, the dip in U.S. sales early in 2018 starts to make sense.

Adding those sales to our chart answers a lot of questions, especially when you consider that it probably takes several weeks for Bolts to go from Michigan to South Korea.

Chart resembles the U.S. sales chart through 2017. But when global sales are added, 2018's sales appear to be rising much like 2017's.

Data sources: General Motors (U.S. and South Korea), InsideEVs (U.S. monthly estimates for April, May, and June of 2018), Automotive News (Canada), and CarSalesBase com (Opel and Vauxhall). June 2018 figures for Canada and the Ampera-e were not available at press time; the figures included in the chart for June are the average of the prior three months' sales for those two categories.

GM said that total global sales for the Bolt were up more than 35% in the second quarter, versus the second quarter of 2017, and that total sales were up more than 40% in the first half of 2018 versus the first half of 2017. Our chart might not be capturing all the markets in which GM is selling the Bolt, but the data we have suggests that if anything, GM is understating the Bolt's sales growth somewhat.

There's one more factor that might be impacting Bolt supply in a small way now, but it will almost certainly become a bigger deal next year. GM Cruise, the company's self-driving subsidiary, has developed a self-driving vehicle based on the Bolt. It's not in full production yet, and won't be until sometime in 2019. But there are hundreds of them operating in test fleets now, all made in that same Michigan factory.

Long story short: Supply is a big factor in the Bolt's U.S. sales totals

It's easy to assume that Bolt sales are suffering because of competition. That might be true to some extent. But it's also true that supplies of the little electric Chevy have been tight, thanks to demand outside the United States.

The takeaway: U.S. sales of the Bolt may be down, but from GM's global perspective, it is doing just fine.