So are you saying that with the Herpstat probe sitting in the exact spot that the snakes would be sitting at and the Herpstat set at 94, that the hot spot isn't actually 94 degrees? This seems odd to me, but I guess I'll have to pick up a IR Thermometer to test it to be sure.
Also do you have the air temp set to 90 or the surface temp of the tub?
Oh, I assumed that the herpstat probe was directly on the heat source like most people do it. How is it attached inside the enclosure if you don't mind me asking?