If it's like mine it's the greatest thing in the world most the time. But on some surfaces it can vary a bit, how you hold it, where you point it.
You SHOULD take readings various places around the tanks and maybe hold it a slightly different angles and get used to it.
Since it works basically by reflecting off the surface back to the unit various things can affect it a little. Just like Radar.
If it doesn't seem to read the liquid correct for you, then you could just float something in the tank like a piece of metal or glass tube floating like a fishing bobber, the bobber should be at tank temps so shoot the bobber for a reading
It should work fine though normally, but like pointing at an angle onto a shiney surface can be a little like looking into the mirror, the angle you look in affects what you see.
I think my Laser tach is more effected by such things than the laser thermometer though.