Are you sure of the temperature of the tank? How are you measuring it? A lot of thermometers, even the liquid crystal, can be several degrees off. The "pre-set" markings on heaters are a total waste, I have yet to find one that's even within 7-8 degrees of what it claims to be set at.
I used to have a photography thermometer, which was guaranteed +/- .5 degrees. Tha's POINT FIVE, half a degree. I double checked my liquid crystals, which all agreed with one another when I picked them up at the store, and they generally read two degrees warmer than the photography thermometer. I suspect variations in the glass thickness, room temperature (because they are mounted externally), and poor quality control in manufacturing all contributed to this.
As far as heaters, the only ones I can't say are definitely inaccurate are those with externally mounted temperature controls (the sensor and controller are not in the tube with the heating element). I haven't tried any of them yet, as the cost is too high for my liking.
Happiness may be the door to Heaven,
But Pleasure is not the Key.