I keep a pretty good eye my server so I know who's playing and how many are playing.
However I noticed that the HISTORICAL DATA at GameTracker is off usually by 3 or 4 players.
For example:
The last couple nights my server was maxing out at 8 players.
Where as GameTracker HISTORICAL DATA was only showing 5.
Not sure if it's a flaw or not?

The historical data is an average over the past hour, so it may not show 8 players unless the average of all the scans in the past hour rounds to 8. It does not only show the max number of players in that time.