"Personal Computer" has long been specifically associated with Windows machines and specifically excludes Mac and Linux. It started with the "IBM Personal Computer" and its branding caused non-IBM machines (eg Macintosh) to use other branding ("a mac"). When IBM partnered with Microsoft to make home computers, this branding really solidified that "PC = Windows".
And then of course, there was the very successful advertising campaign from Apple where they specifically distinguish Macs from PCs: https://en.wikipedia.org/wiki/Get_a_Mac
Essentially, Macs are "personal computers" but they aren't "Personal Computers", if you know what I mean.
I have been rather miffed that the lack of corporate branding/identity for Linux at the time when Macintosh decided to "distinguish" itself now means that GNU/Linux machines are apparently not "personal computers" anymore.
But if PC = IBM PC, and those basically always came with Microsoft software, I guess history supports that interpretation.
