The wet bulb thermometer is used for relative humidity/dew point information. The dry bulb is used for temperature. It's normally in a little ventilated box, but we took it out on the ramp that time. I don't know the tolerances for aircraft OAT gauges, so they may have all been within tolerance, but they were all slightly different. They are also highly susceptible to parallax error, so the indication depends on the angle of the eye to the degree markings, which are rather close together, & only indicate even degrees.
My only point is that OAT gauges aren't precision instruments. I'd much prefer to trust a digital gauge.