Mac ≠ PC ?

Hello Readers,

lately I recognized that The Verge, some other technology blogs and technology related YouTubers differentiate between Macs And PCs.

Some examples of The Verge:

'Minecraft' developer's new game 'Scrolls' out in beta next month

Initially Scrolls will be available for PC only, but a Mac, iOS, and Android version will follow.

Redbox Instant unlimited movie streaming launches publicly

Users can stream movies via their iOS or Android device, as well as their Xbox 360 and Mac / PC (via a Silverlight plugin).

And even Apple differentiates between PCs and Macs.

Introducing the new iTunes.

For Mac + PC
Buy music, movies, and TV shows on any device, and they'll be instantly accessible in your iTunes library on your Mac or PC.
iTunes Match requires [...] or a Mac or PC with iTunes 10.5.1 or later.

Yes, Microsoft does it, too.

SkyDrive and Office work together - Microsoft SkyDrive

With SkyDrive, you get automatic access to your documents from any PC, Mac, iPad, or phone.

PC is the acronym for Personal Computer - a Mac computer falls into that category. The majority knows that a Mac is a PC.
So why do people differentiate between PCs and Macs?