PCs are the new UFOs
This sort of thing happens a lot: a general definition becomes implicitly more specific just from being used just to mean one thing. For example PC. Originally PC meant "personal computer". This referred to many devices like BBC Micros, Amstrad CPCs, even Commodore 64s to some extent. But then one architecture took off and PC slowly got more specific. There was a transitional time when the phrase "IBM compatible PC" was popular but eventually they were all IBM compatible. So now PC means something based on x86. The most common use for PC now therefore (as opposed to computer) is just to differentiate an x86 machine from a Mac.
Which is a problem for one and a half reasons. Mac OS can now run on x86 computers and "PC software" can now run on x64 computers (only half a reason since x64 is designed to be compatible with x86). So what does PC mean now, a computer running Windows? Generally it does I suppose but you can a PC with Linux on it. Would you say you had a PC with Mac OS on it? No, you'd say you have a Mac. So apparently a PC is a desktop* computer that isn't a Mac.
* Or maybe laptop. But probably not a server. So at least the "personal" part of "personal computer" still makes sense