I noticed that my tub temp will climb above the heater set point in the summer as well. note that things don't heat up just because they are insulated or covered, they heat up because there is a source of heat and the insulating cover helps trap that. three sources of heat are:
1. the sun. even with an insulating cover, the sun baking down on the exterior of the tub imparts energy into the water in the form of heat
2. the cycling circulation pump. you don't notice this in cold weather because pumps transmit very little energy to the water, but they to contribute a non-zero amount which will tend to raise the water temperature.
3. ambient temperature. if there is very little temperature difference between the air and the tub water, then it takes very little energy to heat the water hotter than the ambient air.