I'm going to second the fact that you absolutely MUST let any thermometer probe sit in the spot you're measuring for at least 20-30 minutes for it to be accurate. If you just set the probe down and look at the temperature reading, even within 10 or so minutes it can be off by 20-30 degrees in either direction.
I have 4 digital probe thermometers, 3 are the yellow ZooMed ones that I bought at Petco for around $10, and the other one is a black Petco brand one that is exactly the same thermometer as the ZooMed one, it's just black instead of yellow, and I believe that it cost $1 less than the ZooMed ones (the only pet shop within 40 minutes of me is a Petco, no others, and I found the black Petco brand one back in the aquarium supplies as they were out of the ZooMed ones, but it works exactly the same way). I've never had any issues with any of the 4 thermometers, and one of them even had the wire chewed through by my Senegal Parrot, so I had to strip the wires a bit and then solder them back together and then tape them up, and it was fine. I'm currently using 3 of them and they are all pretty close to each other (all 3 of my vivs are the same size with the same UVB tube and the same wattage halogen indoor flood bulb in each).
It's possible that you could have gotten a "bad" ZooMed thermometer I suppose, it happens, but in general they work very, very well. I'd take it back and exchange it with another one, as there shouldn't be any way you'd get a second bad one. I've looked at both Walmarts near me (I have 2 Walmarts literally within 3 minutes of each other, but only one pet shop within 40 minutes, lol) for the Accurate probe thermometer I've seen many people recommend, but neither of the Walmarts, not my Lowes or Home Depot have them, just the regular indoor thermometer with no probe or the indoor/outdoor thermometer that has a big plastic wand, not a probe). So I've been forced to use the ZooMed ones and I've never had any issues at all with any of them, not the Petco brand replica.