to answer your last point fluff knows!
To answer the first point, the filter relys one a list of words, this list is not complete yet so some words will get through, if you see any that really should be filtered but are not then inform the Ed and they will get added to the list and will be filtered. some words are changed to less offensive ones (like fluff) and some are simply stared out, to be fair it would be a huge task to think up a suitable replacement word for every single swear word, so only the more common ones will be replaced by other words.