Likely 32-bit file access[1] and the caching it included? The arrangement where Windows calls 16-bit MS-DOS entry points to do I/O (including from 32-bit programs) to respect preexisting DOS drivers, network redirectors, etc., but then in 3.11+ intercepts those 16-bit entry points (including when used from 16-bit DOS programs) to use a different 32-bit I/O stack when possible... it wasn’t intuitive back then and I don’t think it any more intuitive now.

[1] https://en.wikipedia.org/wiki/Microsoft_32-bit_file_access