Automatic Accepting as "Tentative"

[Jeff]

When someone sends me an invite it automatically goes on my calendar as
"tentative". I don't need to open, read, or accept it. Just as long as it
hits my Inbox it happens. I have a co-worker for which this doesn't happen.
He actually has to open the invite for it to show as tentative. Couple of
questions please:

What is the normal behavior?
What would cause it to deviate from the normal behavior?

Running Outlook 2003 SP1 on Windows XP...

Thanks, Jeff