[Mono-list] io-Layer not abstracting certain calls - Why?

[bvv]

Found an "IFDEF USE_WIN32_API" that chooses between 
CreateFileMapping and mmap in rawbuffer.c.

I was thinking that such APIs that differ between OSes would be 
abstracted out by the io-layer.
In other words any particular reason why we couldn't have had just 
a CreateFileMapping call then went to the io-layer where a 
decision was made to either use windows CreateFileMapping or 
Linux's mmap!

Thanks