At least for me, it's hard to know without knowing more about your setup. One thing for sure, something's going wrong. If your temperature controller is adjustable, have you tried turning it down? What happened then?

Some digital thermometers have calibration adjustments. It may rally be off, or the heater my b off.

Thanks folks! I tried placing the probe of the digital heat mat thermometer directly into one of the flats & that fixed my problem (ie, gave me an accurate reading), after I'd cooked several flats of seeds, of course...........

I've never had an issue with putting the probe in a separate plastic cup of wet growing media, in years past....... Hmmmm........