CONFIRM vs HOLD shortens pending queue?

I'm running tmda 1.1.7 and tmda-cig 0.14

For the first month or so after starting to use TMDA I had my "unknown" 
senders default to hold.  I would examine my various users pending 
queues and pull out those email addresses I wanted to add to the 
whitelists.  I assumed all the rest was spam I simply would ignore them.

Once it seemed I hadn't "caught" any good email addresses in a while I 
decided to switch the default action to to confirm.

This all seems to be ok except for one thing.

The number of messages that are showing up daily in the pending queues 
of the various users  is now MUCH smaller than it was before.

I would estimate that in my own pending queue the amount of new pending 
email is down by at least 70 percent.

I didn't think that tmda automagically removed the source of bounced 
confirms.  I haven't implemented any of the various methods suggested 
in the howto's or FAQ for dealing with bounces.

Any thoughts on what I should look for?

Tony