I can understand why HP, Lenovo, and others are bent out of shape over this. Yearly sales numbers are an important pissing match in the industry, and nobody likes to be outmaneuvered. But of course what the other PC makers should really be mad about isn't that Apple's being counted as the #1 PC maker, but that Apple's actually selling so many devices. And why is Apple selling so many devices? Say all you want about brainwashed zealots and "marketing," but the hard truth is that Apple products sell because people want Apple products. And what do they want them for? Broadly speaking: computing, media, and communication. The same things people want any computing device for.
Some observers are saying that devices that don't have a "full size keyboard" or aren't primarily used for "content creation" shouldn't count as PCs. But that's as preposterous as saying that because some computers don't have spinning hard disks and aren't primarily used for running spreadsheets shouldn't count as PCs. Both technology and people's uses of technology evolve over time, and it's precisely because HP hasn't done as good a job as Apple at adapting their product lineup to meet consumer need that they're being outsold. For goodness sakes, HP even decided at one point it wasn't going to make PCs anymore. How to they expect to stay on top with an attitude like that?
My mother is a good example. She owns two computers. One is a very nice Core 2 Duo desktop with Windows 7, a solid state drive, 4 GB RAM, and a 22" monitor. I built it up for her, and she likes it a lot. She's a teacher, and she communicates with her students by email, she loves Facebook, and she's addicted to online news. She spends at least four hours per day online. Her other computer is an iPad. Even though it's pretty much inferior to her desktop computer in every way, she uses it for 3/4 or more of her daily computing. And that's because of its one key superiority: form factor. She doesn't even value its portability all that much. It rarely leaves the house. But she sits on the couch, half watching TV, half browsing with the iPad, like millions of other, mostly younger, people.
What my mom does on her iPad is completely interchangeable with what she could do on her desktop. Sure, each form factor has its pluses and minuses. She makes certain sacrifices to use the iPad, and reaps certain benefits. Same goes for my daily use of a Macbook Air, compared to my Windows 7 desktop, compared to my 5" Android tablet, compared to my iPhone. We're going to have to get pretty comfortable with the term Personal Computer getting mighty blurry over the next ten years.
So let's break it down:
Personal: something that's designed to be for one person's exclusive use
Computer: electronic device for storing and processing data
A personal computer is a computing device that's designed for one person to use, rather than, for example, a server that's intended for many people to access simultaneously. It can run many operating systems, have many form factors, and even be used for very narrow purposes. Get used to it.