What is your room temp? Sometimes on off types can swing the way you are describing due to slow probe response not just a longer range around the set point, This is often made worse by cool room temps as the tub cools faster than the probe and add that to a slow sample of the unit to the probe and you have a 5-10 degree swing.

Typically I'd never want to see higher than 92, 94 MAX, anywhere inside the tub. I find I loose a degree to the tub itself so I have my thermostat set for 91 and it delivers 90 inside the tub which is about what I am looking for.