Why isn't BAYES_99 + BAYES_999 trusted?

[Steve Bergman]

Is there some reason that the BAYES_99 + BAYES_999 score does not add up 
to 5.0?

I'm trying to "trust the defaults". But what would be the hazards of 
leaving BAYES_99 at 3.5 and upping BAYES_999 to 1.5? It seems that I 
should be able to trust Bayes to declare a message spam on its own.

-Steve Bergman

----
Spamassassin 3.3.1
Spamass-milter 0.3.1-9
dovecot-antispam 1.2+20090702-1ubuntu0.10.04.1
Ubuntu 10.04 LTS