If your husbandry is correct, then it could just be that your BP feel's more secure on the cold side. Do you have 2 hides? Also, your BP is just regulating it's body temperature.

My BP is the same way, husbandry, temps, hides, etc. are all good. And my BP likes to stay in the cool side more then the hot side.

As long as your snake feeds regularly and doesn't seem/look stressed. Then you shouldn't be worrying.