Bayes Expiry - spam gets through

[Jeff Mills]

I have an issue with the expiry of the Bayes databases on my servers.
Every time an expiry is done, quite a bit of spam is let through.
Obviously its removing tokens that are not very old.
I have set bayes_expiry_max_db_size to quite a high size to try and help
this, but still when it expires, spam gets through.

bayes_expiry_max_db_size 4000000

The example spam I have been sent both came through server 2, but this
could just be coincidence.
Server 1 usually gets more mail because it's the first in DNS.

Server 1
Bayes Database Information Number of Spam Messages:	843,399
Number of Ham Messages:	175,621
Number of Tokens:	4,216,073
Oldest Token:	Sat, 10 May 2008 07:58:32 +1000
Newest Token:	Fri, 04 Jul 2008 11:14:26 +1000
Last Journal Sync:	Fri, 04 Jul 2008 11:05:51 +1000
Last Expiry:	Fri, 04 Jul 2008 10:58:27 +1000
Last Expiry Reduction Count:	49,773 tokens

Server 2
Bayes Database Information Number of Spam Messages:	423,059
Number of Ham Messages:	170,864
Number of Tokens:	2,100,113
Oldest Token:	Tue, 06 May 2008 05:34:49 +1000
Newest Token:	Fri, 04 Jul 2008 11:17:36 +1000
Last Journal Sync:	Fri, 04 Jul 2008 11:05:37 +1000
Last Expiry:	Tue, 01 Jul 2008 22:26:32 +1000
Last Expiry Reduction Count:	30,177 token

I would have thought that keeping tokens a month old should be enough to
catch recent spam due to spammers changing tactics so often.

Do others have the same issue when expiring bayes?
Is there a known best setup with regards to bayes setup and expiry?