Why does my thermostat trip every time it reaches the same temperature?
3iE Energy-Monitoring Thermostat
The thermostat continuing to trip at the same temperature point could indicate that your system was cut during installation.
The resistance for your mat or cable is calculated on the length of the heater. Shortening the heater would increase the resistance and thereby the temperature. The cable can operate at slightly higher temperatures for a time, but it may strain the end-splice, therefore the thermostat may then trip and eventually fail.
Check with your installer and test the resistance of the heater. Refer to the installation manual for a list of heaters and corresponding resistances and installation guidelines.