We have a TON of anti-spam systems in place, but it's impossible to stop them all, and once in a while the really sneaky ones slip thru. Usually pretending to be a legit member and then hitting with a bunch of spam.
It's a tricky balance between keeping spammers out, but letting legit members in. If the balance isn't 100% perfect (which is actually impossible) you either block legit members or have opportunity for a really smart spammer to slide in.
It's a constant game of cat and mouse... er, maybe cat and rat!