I just did some testing with two different thermometer as I have recently had very low reading thinking maybe the thermometer is broken.
Had A in mouth and B in the armpit. Mouth 0.1 C lower. Waited a bit before swapping and got same result. So both thermometers are working I guess and the reading is very consistent and not varying a lot for me.
Did not drink or eat before testing. I guess I can trust the temp taken from armpit in the future too as I hate taking it from mouth. Just annoying.