Your heater (any heater) is calibrated in the factory for a given quantity of water at a certain ambient (room) temperature. If your water movement, ambient room temperature, or water volume isn't exactly the same, the temperature dial on the heater won't function the same way in your tank as it did when calibrated. Consider the temperature setting dial on aquarium heaters to be a rough guide at best, and find yourself a good thermometer to use when setting the heaters in your own home.
Also, if your house changes temperature with the seasons, be sure and reset your aquarium heaters when the prevailing weather changes as well.
Also, if your house changes temperature with the seasons, be sure and reset your aquarium heaters when the prevailing weather changes as well.