The two files in "files.zip" have the same contents:

    >>> open('source.css', 'rb').read() == open('destination.css', 'rb').read()
    True

Maybe there's something peculiar about the stat results. Check that they're both reported as regular files with the same size:

    import os
    for f in ('source.css', 'destination.css'):
        print(f, os.path.isfile(f), os.path.getsize(f))