The xmldiff tool is supposed to append Nones to the element with fewer children, but it appended it to the one with more. This means that in the case of
len(children_a) == 5 and len(children_b) == 0,
the code to "pad with nulls" did
len(children_a) == 10 and len(children_b) == 0,
so the zip() loop never ran (thus no difference between the XML trees was reported).
I still need to fix the unit tests.