Since network connections can temporarily drop due to network interruptions, the networking stack holds onto the old IP addresses of LabQuest devices for a period of time, even if LabQuest is turned off or the connection is intentionally disconnected. The list in the networking stack can be several minutes old.
Use the refresh button in the My Lab Setup Sources list to help ensure you only see LabQuest units that are actually available for use with the LabQuest Viewer.