Reoccuring mismatch in free/bury information, cause?

I am trying to find the cause that makes this situation happen. We know how
to fix the problem( /cleanfreebusy ), but the problem keeps occuring and its
starting to make some people angry.

We have a situation where Outlook Calendar data between 2 or 3 people will
not match. For example user A will have a meeting in his calendar for 11am
in the boardroom. User B will want to schedule a meeting for a training room
and invite User A. User B looking at User A's calendar inside of a meeting
request sees that User A is free at 11 even though at User A's desk his
Outlook shows he has a meeting at 11.

User B not knowing that User A is busy at 11 invites him to attend a meeting
and thus creates a scheduling conflict.

We have been correcting this problem by checking that the users are running
the same version of Outlook and/or servicepack (usually they are), but the
fix is usually to run Outlook.exe /cleanfreebusy


So.... we can fix the problem. BUT we would really like to know why this
problem keeps happening. What is the cause of this issue? What triggers
this to happen? We have seen this happen to people that we have previously
fixed. It keeps coming back.

Our Enviroment.
Ad 2k3 Native, Exchange 2k3 SP2 Native
Single Mailbox Server, Single Domain

Multiple Outlook Versions:
Majority Outlook 2003 w/ sp3

others:
Outlook 2002
Outlook 2000
Outlook Web Access users.