I am surprised the IR thermometer is that accurate. Were you aiming it at the thermostat or the water? I would think it might not read correctly pointed at clear water. I have a cheap one from Harbor Freight, but haven't tried it yet. Some people imagine they must be accurate because digital, but I know how they work. Cheap ones traditionally just measure the intensity of emitted IR light, which varies both with surface temperature and the emissivity. They are most accurate measuring a flat black surface. Maybe even the cheap ones today use the 2 color method or laser reflection to compensate for emissivity.
