I hope you're measuring those temperatures inside the tank on the glass. Just because the mat itself is one temperature, doesn't mean the inside is the same.

Glass does NOT hold heat well, instead it usually allows it to pass through. Therefore, insulation of some sort is needed to keep the temps stable inside (important for snakes). It not only helps with ambient air temps but warm and cool side as well. For optimal heat stability, and humidity, you should also have the screen top covered with either plexiglass or the foil treatment.