I am seeing this as well
 

I am running Exchange 2003,
Outlook 2007 ( on all systems)

I have a situation where a user sent a meeting request to a
distribution group (all users)
She has recieved ~26 meeting responses. only 20 of them are showing up
in her "meeting acceptence tracking"

It is my understanding that 2007 does not require you to view the
meeting response to tally it correctly.

Any Ideas?

All systems and servers are fully up to date.

-Ryan