Usually when a word is filtered, anything containing that word will then also have that word filtered out of it. Mother****er, for example. But sometimes that causes issues.
https://en.wikipedia.org/wiki/S****horpe_problem
So I believe in some circumstances, the word is filtered in such a way that other words containing it won't be, so very useful words like S****horpe can be used without filtering.
My guess is that was used here so people can still use words like bitched and bitching without them being filtered. Bitches is interesting, because it's a form of both the noun and the verb, so it's not so clear whether or not it should be filtered.