In the early days of home computers (as makers such as Atari and Amstrad fell by the wayside) the market came to be dominated by IBM (or IBM-compatible) PCs, running Windows, and Apple Macs, running Apple's own operating system. (Apple simply decided not to call their devices PCs, just to make them stand out from the crowd).
Eventually 'IBM-compatible PC' just got shortened to 'PC', so that the term came to relate only to machines running Windows (or, more strictly, to those running non-Apple software, since people also refer to PCs using Linux).
Further, manufacturers tended to drop the 'PC' bit from 'laptop PCs', simply calling them 'laptops', so that many people now think of 'PCs' as only referring to desktop computers (running Windows or Linux).
There's very little scientific reason behind it; it's simply the haphazard way that language evolves.