Hello and welcome to MFK!
I made a similar post when I first joined. Out of the 13 or so thermometers I have, there seemed to be some variance in all of them. The stick on strips are very convenient, but not so accurate. Most likely your "room temperature" will be effecting the results slightly since the strip is on the outside of the tank.
My digital thermometer reads about 2 degrees lower than the actual tank temperature.
The best results I have had are with the old school mercury thermometers with a small suction cup that attaches inside the tank. Even with these, there is about a 2 degree temperature difference total when compared together.
For the most part, 2 degrees isn't going to make a big difference in your tank. I have all mine set to run 78 degrees, so at worst case, the temp is actually 76-80. Temp swings are far worse than being off a degree or two.
I would expect that any heater that has the temp settings formed into the housing mold aren't going to be too accurate either. They would be more of a reference point. Also, the heater sensor is only going to sense the temperature directly around the heater.
Unless the equipment is very high quality, I wouldn't expect much more than 2 or 3 degrees accuracy. And I wouldn't be too concerned either. I would buy an in tank thermomemter though and you could compare the two.
