Because when you are configuring a forum like this one, you enter a list of off limit words. Words which are not on the list get through. Twat would not be a good one to put on there; for one thing, it's not that offensive. For another, if it appears in any form, including inside another word or as a typo, it would not appear. That coudl be inconvenient.
At least, this is what I think is the deal, because I have a php forum that is basically a whole lot like this one (different skin) and I had to provide it with a list of words I didn't want to go through.
