The file command in msys is old (v5.04 vs 5.11 in cygwin). This has a bug when identifying a 64-bit dll, describing it as "PE32+ executable for MS Windows (DLL) (GUI) Mono/.Net assembly" instead of
"PE32+ executable (DLL) (GUI) x86-64, for MS Windows".

This makes the msvcr100.dll verification fail.

While the msys file command is incorrect, the relevant part here is that it is indeed classified as PE32+. For msvcr100.dll, only the 64-bit version is PE32+, and this is enough for us to identify it.