The data collection is not instantaneous and has a delay of up to ~5 minutes to detect a join/leave event. This should largely cancel out for longer sessions but may become an issue if a session is shorter than the collection interval

The service that runs collection, as well as gametracker itself, are both subject to downtime. In all, the service has been up for >99% of the last 30 days, which means that about 6 hours is unaccounted for. Some of this downtime may be server downtime, but it's impossible to tell the cause of any specific downtime period.