Wow64 and native x86 versions of a DLL

Wow64 includes a complete set of 32-bit system DLLs implementing the Win32 API (for use by Wow64 programs). So, what’s the difference between the native 32-bit DLLs and their Wow64 versions?
On Windows x64, not much, actually. Most of the DLLs are actually the 32-bit copies from the 32-bit version of the operating system (not even recompiled). For example, the Wow64 ws2_32.dll on Vista x64 is the same file as the native 32-bit ws2_32.dll on Vista x86. If I compare the wow64 version of ws2_32.dll on Vista with the x86 native version, they are clearly identical:

32-bit Vista:
C:\\Windows\\System32>md5sum ws2_32.dll
d99a071c1018bb3d4abaad4b62048ac2 *ws2_32.dll
64-bit Vista (Wow64):
C:\\Windows\\SysWOW64>md5sum ws2_32.dll
d99a071c1018bb3d4abaad4b62048ac2 *ws2_32.dll
Some DLLs, however, are not identical.
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