Regarding ACPI specifically, not everyone uses the Intel developed one - Microsoft supposedly implemented their own, as well as OpenBSD (See section 3.2 and on of this: http://www.openbsd.org/papers/zzz.pdf)
ACPI is notoriously broken in many places - the OpenBSD dev's frequently had to do a lot of "bug for bug" hacks to talk to the hardware just how Windows did, in order for things to work.
My understanding is that the windows acpi code is actually very similar to or derived from the intel code.
As for bug for bug compat, that's really more an issue of broken bios. I.e., the acpi byte code is broken, not the implementation that interprets the byte code. Windows doesn't necessarily do anything crazy, it's the bios that asks "is this windows?" And then shits itself if the answer is no.
ACPI is notoriously broken in many places - the OpenBSD dev's frequently had to do a lot of "bug for bug" hacks to talk to the hardware just how Windows did, in order for things to work.