I tested the controller and when I set it to the certain temperature I noticed it never stayed on that temperature when it's once it got up to it. For example when it was set to 70 it would keep bouncing between 63 and 74 degrees somewhere in that area but never stabilized on 70 C.

It is usual for the temperature to fluctuate a little when the mat has reached the set temperature. 

The mat heats up to a little higher than the set level; the temperature sensors in the mat detect it and send information to the central controller. The controller carries a signal back to the mat to stop heating.

When the temperature decreases a little, the temperature sensors in the mat detect it and send information to the central controller. The controller carries a signal back to the mat to resume heating again and so on.

It is the standard operation scheme of the thermostat type controller and temperature sensors in the mat.

The hot crystals radiate and dissipate heat fast. The airflow velocity may be another reason for cooling the mat surface and interacting with thermostat functioning. If you change the mat thermal insulation condition, such as standing up from the mat or removing a blanket or protector, the temperature may drop even 10-20 C.

If you set the controller on the highest temperature, even though the temperature indicator on the controller will finally go up to 65-75 C, the mat will be only slightly warm to touch.

It will get warmer after your cover it again or lie on it.

When you lay on the mat, your body presses the stones to the heating layer, where the sensors measure the temperature, and the mat surface becomes warmer.