Maybe there is some ambiguity in the description which have permitted to many to pass the kata. In life there are always ambiguities... To include "your" tests I should change the description and I think that nobody - even only a bit dishonest - changes the rules when the game is on.
Moreover - can be seen as a bad excuse - I would have to modify the kata in its 32 languages and I don't have enough time for that:-)
Sorry if I disappoint "your" point of view but mine is I can't change anything and, after all, it's my kata.

Interesting suggestion but I did not say explicitly in the description that "a" must be different from "b". To include these test cases, I should have made it clear somehow. Sorry.