Sneffy
Maybe contacts shown on Mac are not comming from AD, Jabber can also show you email client's contacts.
Look for "view profile" for one of the contacts and compare it with the same "view profile" contact on a Windows client, is the IM address the same?
Also if the same user that has the issue on the Mac logs into a PC, does he/she experience the same problem?
Deleting the contact, emptying the cache, re-opening the client and adding the contact again might help.
Cheers,
Oscar