How do you know the therm is "off"? Maybe it's not the therm that's off- maybe your heater isn't calibrated. Are you comparing temps with another thermometer or with the "number settings" on the heater?
Why is the probe in the filter box? Do the instructions say to put it there? (not putting you down, I really don't know.)
It seems to me that to get an accurate reading of the tank temp, you should put it in the tank. It could be that your therm is correctly measuring the temp inside the filter box. Remember that the relatively small filter box & it's small water volume will hold ambient heat more easily than the relatively large tank- even with the constant water turnover.
Try putting the probe in the tank and compare again after an hour or so.
HTH
Oh yeah, please stop yelling, too.