Do any of these Macs have a spotty connection to your JSS? I may be mistaken, but I believe if a Mac can't see the JSS at logout/login, it will pretty quickly skip any policy checks and continue the logout/login. If on the other hand it can see your JSS, but the connection is bad or communication is hampered by things like firewalls, etc, you may get a delayed logout or login. In other words, its best if it can clearly see or not see your JSS. If its in the middle, you can get weird results.
