[Mimedefang] mimedefang letting some spams through...why?

[Tony]

server = raq550
running on strongbolt 2
spamassassin.i386 0:3.2.5-1.el4
mimedefang.i386

I have mimedefang set to 'action_bounce' emails that are labelled as
spam by spamassassin. But I'm confused why mimedefang has let 3 emails
through to my inbox even though they're labelled [SPAM] in the subject
line and with a spamassassin score of >10. It doesn't make sense because
my daily mail log shows 10 emails rejected by mimedefang because they
were clearly spam so mimedefang appears to be working properly. Why
would it bounce some and let others through?
One thing I noticed was the emails that got through - clearly labelled
as [SPAM] were sex/erotica spams all with a score of > 10.
Thanks for any advice.