Forehead thermometers have to be doing some kind of calculation to determine your internal temperature based on only seeing your forehead skin temperature. That calculation has to be based on some set of assumptions.
I think those assumptions are probably true for a lot of people a lot of the time, which means these will give accurate readings a lot of the time. But the assumptions might not hold true for me, or for some individual situation I’ll be using them in. I’d rather just measure the temperature under my tongue and be sure.