Out of the 21 million "PCs" for Apple in this list, 17 million are iPads. So Apple really sold 4 million PCs compared to HP's 13 million.
Definitely an interesting trend but nowhere near the "No. 1 in PC sales", at least according to what most people would think of as a PC.
I fail to see why people keep bringing it up as if it was relevant to anything.
That said, it's going to be hard to define what tablets qualify as PCs very soon. --With Windows 8 I would place ARM tablets in the same category as the iPad, but I would place the Surface Pro in a different category - maybe not PC, but definitely more powerful than a tablet.
This is a way to pump up Apple stock with intentionally misleading information. No one considers an iPad a "PC" unless they're being pedantic or trying to mislead.