Some spam mails do not trigger anything?

[Remco Barendse]

Hi list!

I'm a bit puzzled.  Every day some spam mails are getting through without 
triggering anything in SA. I'm using SpamAss 3.1.7 with DCC, Razor and 
Pyzor2, and I have the following lists in my MailScanner.conf:
Spam List = ORDB-RBL SBL+XBL spamcop.net NJABL

I got an e-mail this morning for example :
X-nu-MailScanner-SpamCheck: not spam, SpamAssassin (not cached,
    score=4.726, required 6, BAYES_99 3.50, EXTRA_MPART_TYPE 1.09,
    FORGED_RCVD_HELO 0.14)

This one triggered BAYES_99 but some do not even hit that.

Is anyone else seeing this? I am logging all spam messages to a mail 
account and I can see that all things seem to be working.

Also is it correct that only the first MailScanner checks the mail for 
hits on ORDB-RBL SBL+XBL spamcop.net NJABL etc. and subsequents passes 
through MS on other servers do not do this check again?

Thanks :)