Message Number Jumping

I provide hypertext versions of three different mailing lists, and 
just noticed that the message number assignments seem to be 
accelerating!  For example, in one list 
(http://www.itsonline.com/itsl/threads.html) the message number 
jumped from msg00074.html to msg00638.html in one two-day period!

Any idea of what's going on here?  Admittedly, I haven't cleared out 
my "mbox" in a while, and MHonArc runs from a cron job.

Thanks for any ideas.

Jerry Werner