Without calibrating the temp sensor and gage against a known sensor, we probably don't know what temperature "out of normal" on the gage really is. It could be just 10 F.
One thing to understand is that the thermostat does not control exactly at its setpoint (180 F). The engineering concept is "proportional offset", which you can find in wikipedia or other sources on "proportional control" or "PID", if interested. The higher the thermal load, the more the thermostat must open beyond its nominal position. This requires a higher than normal temperature. Your experience may be normal, based on the simple controller design.