Interesting issue with mapping accounts

On my juniper gear I have 3 tiered profiles (tier1,2 and 3) each with varying permissions (tier3=SuperUser and tier1=readonly). Using LDAP I map specific users to these profiles so when bob.smith logs in I pass the the VSA Juniper-Local-User-Name = tier3 from the radius server and bob.smith logs in. Accounting data is logged as bob.smith too. Here's the wierd part. If bob goes home in the evening and doesn't log out of a router, the accounting log shows bob.smith (STOP) after a certain amount of time however the accounting log show 'tier3' still logged in. Any ideas? N