Stove temps & gauges accuracy

I have a 150 in a 24 by 32 shed that is not all insulated. With my water setpoint at 180 the gauge on the side reads about 160. This is my second gauge as I bumped the first one and broke it. The first one did not read that close.

I vary my temp. setpoint with weather conditions. I'm currently on my third Press/Temp gauge located on the side of my 100.........that's another story. This gauge is the same as the one located about 4' downstream from the supply exiting the top of the boiler. Rarely do all the gauges temps match. The side gauge is generally 5-10 degrees lower and the two others are usually within a couple of degrees. None of the 1 1/4" copper is insulated but the boiler is in my insulated attached garage. I have a temp gauge on the return behind the boiler which can be calibrated. Is your temp gauge installed in a well ? If it is pull it out and check the bottom for an adjustment screw.