help with cyr_expire please

[Per olof Ljungmark]

It seems I do not fully understand how cyr_expire works. Some background:

I've set up a testing server with 2.3.12-p2 with delayed expunge and 
delayed delete. In cyrus.conf:
delpruneandexpunge  cmd="cyr_expire -D 7 -E 3 -X 7" at=0400

and it all worked as expected.

Now I'm ready to implement this on our production (2.3.12-p2) machine, 
added the proper statements to imapd.conf
expunge_mode: delayed
delete_mode: delayed
deletedprefix: DELETED

Then i tested the following command:
su cyrus -c "/usr/local/cyrus/bin/cyr_expire -D 5 -E 5 -X 5 -p 
user.spamdump -v"

and to my horror it did not only delete expunged messages but a fair 
share of messages without any flags set:
expunged 677607 out of 682456 messages from 30 mailboxes

Unfortunately I have no idea why this occured, can someone shed light 
here? Obviously I cannot implement this now... or I'm off very quickly 
to South America...

and, what is the difference between "delpruneandexpunge" and "delprune"? 
The delpruneandexpunge prefix is not documented anywhere AFAICS.

Thanks!