Probably because the votes weren't all that suspicious: two semi-regular HN readers who upvote a story in quick succession, but certainly not simultaneously - I have no idea how you'd like to "catch" those (or even whether that would be all that desirable).

BTW, have you ever thought about making other people's "saved" views (i.e. their upvotes) public (maybe w/ opt-in)? It would be an awesome data set for collaborative filtering ...