The thermometer on the wall is not in the basking spot so won't register as high as the basking spot. A 10 degree difference is fine as you're looking to have a lower temperature out of the basking spot anyway.when the basing spot and uvb lights are off for the night the thermomiters are within 1 degree of each other which is right soo.... the only thing i can think is the thermomiter on the wall is to far away to be accurate enough.
this brings me back o the drawing board because if a perminate themomiter probe is on the spot exactly, ether the beardie lays on it giving a fulse reading or knocks it away from the basking spot.
As for the Beardy laying on the basking spot probe, don't you think everyone would have this issue? Just get the basking spot correct and then move the probe to the side slightly, make a note of the temp and use that as your guide. Then just periodically check the basking spot, simples.