I have one of those 6 dollar Walmart combo units in there along with the aquarium thermometer, and they both read real close to the same thing. I feel better having two in there.
My LG came with the thermometer stuck to a piece of clear plastic that you lay right on top of the eggs. That may be what you are seeing. Actually, the one that came with the incubator was broken so I just taped a cheap aquarium thermometer to the plastic and it seems to be working fine.