By the sounds of it when say notches its the old non electronic mechanical stat that sits in a wet well socket on the water heater...poor accuracy even when new ,, only way the ensure what each notch represents as a temp is to go through a long several day test at each notch,, check the temp at a hot tap with a thermometer.
They can and do go out of calibration over time by usually an increase in temp, not the other way around
Then there is also something called stacking that can set off a t&P valve...