Is the probe directly on the inside of the glass, in the middle of the UTH where the hottest part would be? Also, is your UTH stuck to the OUTSIDE of your tank?
I like the digital thermometers too apparently they are the most accurate.
But you do need more than one. You definitely need a probe for the UTH (and a thermostat, of course to control the heat output), which it looks like you have.
You'll need another one to measure the air temperature as well, and I'm not sure if you have that already?