weird ownership problem

I can hardly believe I'm posting a question this weird, but here goes:

I have a mail server running qmail in a maildir setup.  As a part of my 
anti-spam training regimen, I regularly move messages between 
directories and re-assign permissions accordingly.

What's weird is that when I type a command such as:

# chown -R mailadmin /home/mailadmin/Maildir

It has the effect of setting ownership to mailadmin on messages that are 
*not* in this directory tree.  What's weirder is that it only seems to 
do so for messages that were delivered directly to a different user... 
never for a message that was actually transferred in (and out) of 
~mailadmin/

I have actually verified this by doing something like the following:

ls -al ~user/Maildir/new; chown -R mailadmin /home/mailadmin/Maildir; ls 
-al ~user/Maildir/new

Which shows the correct permissions immediately before the chown and the 
incorrect permissions immediately after.

Possibly useful fact: all users may write to ~mailadmin/Maildir as all 
users use it as a "drop box" for potential spam.

I've worked around the problem but I'd be very interested to learn 
what's going on here.  Any ideas or theories at all?

Thanks,

Dylan