Surprised by stable sort

gistfile1.txt

def two_sort(ex):
    ex_copy = ex[:]

    ex.sort(key=lambda tup: tup[1])
    ex.reverse()
    print "Ex:  ", ex

    ex_copy.sort(key=lambda tup: tup[1], reverse=True)
    print "Copy:", ex_copy

    assert ex == ex_copy # fails

if __name__ == '__main__':
    ex = [('a', 0), ('b', 0), ('c', 2), ('d', 3)]
    two_sort(ex)
    # Ex:   [('d', 3), ('c', 2), ('b', 0), ('a', 0)]
    # Copy: [('d', 3), ('c', 2), ('a', 0), ('b', 0)]