Reports from analysts such as Canalys raised a few eyebrows by saying that Apple reached over 20% share of the PC market for the first time in Q4 of 2012…if you count tablets. Canalys claims that one in six PCs shipped that quarter was an iPad, and that tablets as a group made up one-third of PC shipments in that quarter. In the same quarter, non-tablet PC shipments declined, continuing a long trend.

Many online commenters question the idea that tablets should be included in PC sales numbers. Unsurprisingly, some of the most vocal opposition is from the “specs, desktop, and keyboard” geek crowd who insist that tablets can’t do the job that a “real PC” can, and therefore you can’t count a tablet as a PC. That perspective may be technically sound, but may not be what matters to the market. And thinking of this as merely a specs comparison makes the questionable assumption that tablet sales and PC sales are functionally separate categories. For example, is Microsoft Surface Pro with full Windows 8 a tablet or a PC?

To get a better answer, think about this question from the point of view of a PC manufacturer (or as they say, “follow the money”). If you have a customer who buys a PC from you on a regular basis, but this year they bought somebody else’s tablet, your customer realized that many or all of their most frequent computer needs can be fulfilled with a tablet. That’s quite plausible given that so many of today’s applications are web-based and rarely require the full horsepower of a multi-core PC.

Tablets cut into PC sales to some extent as PC replacements, and to an even larger extent they can delay a customer’s PC upgrade cycle. That means tablets do not have to be technically equal to a PC in order to have a financial effect on PC manufacturers. Tablets affect PC sales as they are.

The bottom line, if you’re a PC manufacturer, is this: If your customer didn’t buy your PC because they bought somebody else’s tablet instead, then a tablet sale has to count as a PC sale…because a tablet cost you a sale.