expunge not removing attachments?

[Ralf Hildebrandt]

I have a large mail backup folder backup at backup.invalid; I'm cleaning
up daily like this:

infimum=`date -d "-4 day" +"%Y-%m-%d"`
doveadm expunge -u backup at backup.invalid mailbox INBOX SAVEDBEFORE $infimum 
doveadm purge   -u backup at backup.invalid

yet I see this:

# find attachments/ -type f -ctime +5 | wc -l
7522
# find attachments/ -type f | wc -l
127579

# find attachments/ -type f -mtime +5 | wc -l
14361
# find attachments/ -type f | wc -l
127793

About 5.9% of the files in attachments and below are older than 5 days.
Why? Is that normal?

using dovecot 2:2.3.1-1 from the official repos.
-- 
Ralf Hildebrandt
  Geschäftsbereich IT | Abteilung Netzwerk
  Charité - Universitätsmedizin Berlin
  Campus Benjamin Franklin
  Hindenburgdamm 30 | D-12203 Berlin
  Tel. +49 30 450 570 155 | Fax: +49 30 450 570 962
  ralf.hildebrandt at charite.de | https://www.charite.de