Racks are not very efficient heating wise. Aside from the mentioned thermometer issues (I have 15 herpstat (probes and all of them are with in 0.3ºF or each other and yes my business owns a very good accurate thermometer and no I would not recommend buying one...) I would trust the herpstat probe but a simple check is to swap them and see if they are reading the same variation.

Racks really want ambient air temps to be quite warm typically 80ºF is what most use. There are strategies to over come this but it is complicated. I have done so because my snake room has seaweed bags in the walls and heating the room is not practice until I tear out the walls inside and out...

I run hot and cool side tape (4 inch and 17 inch) and a insulated rack with T-stats on both hot and cool as well as top set and bottom set. (4 probes) to keep things stable and correct. It is far easier to stabilize room temps and then set a mid point top to bottom.