The massive success of Apple's (NASDAQ:AAPL) iPhone has made the Mac a bit of an afterthought in the eyes of investors. While the Mac is a solidly profitable $25 billion business, Apple is expected to bring in more than $230 billion of revenue and more than $50 billion in profit this year.

Yet Apple's Mac business has stayed strong and even provided some modest growth in recent years. In the March quarter, Mac shipments rose 10%, though Mac-related revenue rose only 2% due to the strong dollar and lower average selling prices.

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Apple's Mac business has grown despite declines in the broader PC market. Photo: The Motley Fool

Investors were hoping that Mac sales kept growing at a healthy pace last quarter, especially given the April launch of the highly anticipated new 12" MacBook. But shipment estimates from two leading market research firms paint starkly different pictures of the Mac's success during the past three months.

Dueling estimates
Last year, Apple shipped 4.41 million Macs during the June quarter. According to Gartner, it didn't do much better this year. In fact, Apple wasn't among the top five PC vendors globally according to Gartner's estimates.

That means that we don't even have an exact shipment estimate. All we know is that Gartner thinks Apple shipped fewer than the 4.56 million PCs it thinks fifth-place, Acer, shipped. This implies that at best, Apple's Mac unit shipments grew by about 3% year over year.

By contrast, IDC thinks that Apple was the No. 4 PC vendor in the world last quarter, edging out Asus as well as Acer. IDC estimates that Apple shipped 5.14 million Macs during the period, representing roughly 16% year over year growth. Obviously, that's dramatically better than what Gartner expects.

Who should we believe?
So which firm should investors believe? Gartner doesn't have much of a track record: based on its market definition, Apple has not cracked the top five on a global basis anytime recently, so it hasn't reported global PC shipment estimates for Apple.

By contrast, IDC has frequently ranked Apple as one of the top five PC vendors worldwide in the past year. As a result, there are publicly reported IDC estimates of Apple's Mac shipments for several recent quarters, which can be compared to the actual figures reported in Apple's earnings results.

For the September quarter last year, IDC estimated that Apple shipped 4.98 million Macs. But it undercounted by about 10%, as Apple later reported that it shipped 5.52 million Macs. On the flip side, for the December quarter, IDC overestimated Apple's Mac shipments at 5.75 million -- the actual figure was 5.52 million again.

In the March quarter, Apple fell out of the top five by IDC's reckoning, which means that the firm estimated Mac shipments at less than the 4.80 million it attributed to No. 5 vendor Asus. Indeed, Apple only shipped 4.56 million Macs during the quarter.

Thus, the data is mixed. IDC's estimates clearly aren't very precise, but they aren't biased one way or the other. Meanwhile, it's not possible to evaluate Gartner's performance based on publicly available data.

IDC's bullish case seems more likely
So it's a toss-up as to who's right about Apple's Mac sales last quarter. That said, I'm putting my money on IDC's more bullish scenario. The popularity of the new 12" MacBook should have helped, despite apparent supply constraints.

Furthermore, while Windows PC shipments were depressed by the impending arrival of Windows 10 later this month, there's no reason why that should have affected Apple's shipment volumes. If anything, a lack of compelling Windows PC options could have steered a few more buyers toward Macs.

We'll find out for sure which research firm did a better job estimating Mac's June quarter sales when Apple reports its results later this month.

Adam Levine-Weinberg has the following options: long January 2016 $80 calls on Apple, short January 2016 $120 calls on Apple, and short January 2016 $140 calls on Apple. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.