When you open the tub, you pull it off the heat source. Unless you're able to stick your arm in and get a temp gun reading where the tub still touches the heat tape, the gun will read really low. It happens with my tubs as well, and I'm using a Herpstat. Acurites, all new, read the proper temps and the gun readings are at least 10 degrees lower.

Soon I intend to make a rod to attach the gun to with a press bar to push the button so I can get readings where the tub is still on the heat pad. THEN we'll see what's going on in there...