Win32 has various different calling conventions, and each function is annotated accordingly in the header files. It's all a bit of a mess, which is presumably why they drastically simplified it in the x64 transition.

(And realistically, for all but the most trivial functions, having one convention is probably a highly reasonable default. Trivial functions should probably be made available to the compiler for inlining anyway. Note also that GCC allows you to override the number of arguments passed on the stack via an annotation on 32-bit x86, if you insist.)