How much circulation do you have in your tank? And what part of the tank did you take the temp from(top middle or bottom)?
I've noticed since I switched from power filters to sponge filters there is a pretty good difference from the top tank water temp to the bottom. Even more so when I didn't have enough air running through them to help move the water, the bottom was like 72 or 73 and the top was at 81.....
If this is not the case, if your heater keeps the temp stable even if it is too high, just re-adjust you dial down until it gets in the range you want it to be at, I do this currently with my 200 watt aquaeon heater, it stays stable its just inaccurate.