mail moved from /var/spool/mail/username to ~username/mbox , only for some users?

Greetings,

Our mail server  runs UW IMAPD on Debian Linux.


For many users, our MTA (exim4) use ~/.procmailrc to move mail to 
~/mbox  when it comes in.     For some other users who do not have a 
.procmailrc in their ~ , it seems that mail stays in 
/var/spool/mail/username until  they connect via IMAP,  at which point 
IMAPd moves the mail to their mbox (according to syslog):
Example from syslog:

Apr 13 11:09:36 caen imapd[28575]: Moved 1638 bytes of new mail to 
/homes/user/mbox from /var/mail/user host= examplehost.com [123.95.17.2]

For some other users still,  the mail stays in 
/var/spool/mail/username,  IMAP functions normally but the mail stays.  
I am trying to figure out why IMAPd moves some users mail to their mbox 
and keeps other users in /var/spool/mail .


Thank you,

Kevin Long