Looking at the ReactOS sources, it appears that Windows doesn't do any rounding, so it's all Python's fault. Windows keeps its system time (in KI_USER_SHARED_DATA) in essentially a FILETIME represenation (i.e. units of 100ns since 1601), and never needs to convert it. So CreateFile just uses the current system time as-is when filling the LastWriteTime.

I don't think it's enough for Python to convert these monotonically. The error would already occur if Python cannot convert bijectively, i.e. if getting the system time into float and back won't get the same FILETIME. It might just be that a 64-bit float value (with 53 bit mantissa) is too imprecise for that operation. A FILETIME currently has 56 significant bits.