The filecmp.cmp function has a shallow option (set as
default) to only compare files based on stats rather
than doing a bit by bit comparison of the file itself.
The relevant bit of the code follows.

    s1 = _sig(os.stat(f1))
    s2 = _sig(os.stat(f2))
    if s1[0] != stat.S_IFREG or s2[0] != stat.S_IFREG:
        return False
    if shallow and s1 == s2:
        return True
    if s1[1] != s2[1]:
        return False

    result = _cache.get((f1, f2))
    if result and (s1, s2) == result[:2]:
        return result[2]
    outcome = _do_cmp(f1, f2)
    _cache[f1, f2] = s1, s2, outcome
    return outcome

There's a check to see if the shallow mode is enabled
and if that's the case and the stats match it returns
true but the test for returning false is for only one
of the stats attributes meaning that it's possible for
a file not to match either test and the function to
carry on to the full file check lower down.

The check above to see if the stats match with
stat.S_IFREG also looks wrong to me, but it could just
be I don't understand what he's trying to do :)

This code is the same in both 2.3 and 2.4