What brand of heater is it? What type of thermometer, Mercury, Digital, Stick-on Liquid Crystal (I'll assume digital since your readings are so precise)?
You do have alot of heat-producing sources in your aquarium. Pumps and lights can contribute to heating your water, especially during the day when your lights are on and the room temperature increases. Also, most submersible heaters are not extremely accurate in their settings. If you think you have your heater set on 78, it will most likely be set at +/- one to two degrees from that.
Are you at home often enough throughout the day to monitor the fluctuations and chart them?