the FortiGate is connected to only one collector. The one collector that is active, will do workstation checks and remove users if they are not logged in anymore.
The Collector that is not active, will not do the workstation checks until it is active. If it was active it will have done until it got inactive.
"Not verified" is not directly related as it means that the workstation WAS done BUT failed. The workstation check was unable to verify the user status. Then the user is put to "not verified". That in turn starts the "dead entry timer" (default = 480 minutes).
Hope this clarifies this a bit.