SPAM filter is terrible

I constantly receive SPAM in my In Box.  Why is Xfinity's spam filter so bad?  I don't have the same issue with Yahoo.  For example, I've been getting 5 to 10 emails, all at the same time with the same subject, all from different email addresses.  They are obviously SPAM.  I get tired of having to create my own filters to pick up what Xfinity misses.  Can't their algorithm be improved?