What kind of t-stat are you using?
I use an accurite w/ my herpstat to monitor the temps in my rack but I couldn't get similar readings for the longest time. I ended up taping the accurite probe to the inside of a spare tub, right over the heat tape. That was the only way I could ever really acheive similar readings. Unfortunately, taping the accurite isnt an option in an inhabited enclosure due to the dangers to the snake that tape can cause.
Are you using an aquarium style tank or a tub? Do you have the accurite placed directly over the heat source or is there some form of substrate between the accurite and the UTH?