You should be able to calibrate the thermometer. Stick it in boiling water for about 20 secs (ie water that is still boiling, not just brought to the boil). If it doesnt show 100 then you will be able to get an idea of how far out your gauge is.
You can adjust many thermometers using a nut directly behind the gauge but when I tried this with my first thermo I couldnt tighten it back tight enough and it tends to move. I also crushed the stem a bit trying to grip it.
Freshness of the bean is another factor that you should consider.