auto update incrementing incorrectly?

[Thomas Anderson]

I've been having problems lately with Viagra spams coming through as
unsure in unusually large numbers.  This is odd because I've never ever
considered these to be ham, but this was my result when classifying one
of them manually:

                                n    pgood     pbad      fw     U
 "subj:Viagra"              15667  0.011043  0.004035  0.267599 -

Then, after a little while and bunch of more Viagra spams passing
through (classified correctly as spam) and auto-updating, this was the
result:

 "subj:Viagra"              15684  0.011056  0.004039  0.267572 -

Why would "subj:Viagra" be getting hammier?  None of them were
classified as ham.  Pgood is almost 3 times bigger than pbad when pgood
should really be zero.  Here is my configuration:

spam_cutoff = 0.50
ham_cutoff = 0.10
min_dev = 0.25
robx = 0.40
robs = 0.22
thresh_update=0.05

Any ideas?  Has anyone else seen behavior like this?

Tom