Ford Focus. Photo source: Ford Motor Company

Ford (F 0.08%) on Tuesday trumpeted the sales success of its popular Focus compact, citing just-released totals from analytics firm R. L. Polk that showed the Focus was the best-selling car in the world in 2012.

Not so fast, said archrival Toyota (TM 1.07%).

The Japanese automaker claims that its Corolla compact outsold the Focus last year, 1.16 million to 1.02 million. Toyota insists that Polk's figures, which count new-car registrations in every country in the world, have undercounted the Corolla by nearly 300,000 units.

So who's right? I have no idea. But it will probably depend on how you look at it.

For starters, it depends on the meaning of "Corolla"
This isn't the first time the two giant automakers have had a spat over this issue. Last August, Toyota howled after Ford claimed that the Focus had been the sales leader in the first half of 2012, based on data from another analytics firm, IHS Automotive.

Polk hasn't (yet) released details of its tallies, so it's hard to say exactly where and why its numbers don't line up with Toyota's.

But it's a safe bet that it will all come down to quibbles over how you define "Focus" and "Corolla".

See, the problem is, Toyota sells several variants of the Corolla in different parts of the world. Some of those variants are called "Corolla", some aren't, and some are called one thing in some markets and something else in other markets.

Specifically, in some places Toyota sells a hatchback version of the Corolla. That car is called the Toyota Auris, except in a few places, where it's called a -- you guessed it -- Corolla. There are also other variations, like the Toyota Verso, which is sort of a Corolla station wagon (and which used to be called "Corolla Verso" in some places).

And then there's the Toyota Matrix and the Scion xB, both of which are – mechanically speaking – also Corollas, more or less.

Toyota has offered up a few different sales totals for its compact standard-bearer(s). But it insists that just by counting vehicles called "Corolla", it squeaks out a lead over the totals Ford and Polk are citing for the Focus.

Confused yet? Think of how the analysts at Polk feel.

Ford's totals are simpler, but not simple
Ford's case is somewhat simpler, but that doesn't mean it's simple. Ford's Focus is pretty much the same all over the world... except in China, where there are two separate cars called "Focus". One is the familiar model as sold here in the U.S. today (called "New Focus"), and the other is the last-generation European version of the Focus, called "Classic Focus" and sold with an entry-level price.

That's common practice in China – General Motors (GM -0.04%) and Volkswagen (VWAGY -1.13%), among others, do the same things with old and new versions of some of their global models. It's also not unheard of to count sales of both cars in one total, as Ford does with its Focuses. That's why Ford's news releases touting Focus sales in China often use the word "nameplate" in place of "model".

But that means that Ford's 2012 sales totals for "Focus" count thousands of cars (all those "Classic Focus" sales in China) that are mechanically different from the "Focus" that is currently offered by Ford all around the world.

Of course, to be fair, this is hardly a practice that's unique to Ford. To cite just one example, Toyota sells three different vehicles called "Prius" here in the U.S. – the small Prius c, the regular Prius, and the large Prius v – but counts them all as "Prius" in its monthly sales reports.

And Toyota hasn't – yet – quibbled over Ford's (and Polk's) inclusion of the Classic Focus in its totals. As I write this, the company's complaint is that Polk missed a whole bunch of Corollas in their tally.

But does it matter?
Really, it doesn't. It's just about bragging rights. Maybe a few people somewhere will be more inclined to buy one car over the other because it was the best-seller last year, but I don't think it's going to make much difference in this year's sales totals.

This much seems clear: The Focus and the Corolla were the two best-selling cars in the world last year.

I think.

Long story short, both companies' compact models did well last year.

But we knew that, right?