Every thermostat I've ever tested (that would be several), there is almost always vary a few degrees from one stat to another of the same brand and temp. Mass production causes that. Could be as much as 5 degrees difference between two thermostats of the same degree from the same company.

That said, different thermostat makers have differing opinion on what a 180 stat should operate at. Some believe the stat should be wide open at 180, but others believe they should start to open at 180. It should be pretty obvious the actual operating temp is going to be different between those two beliefs.

Gauges and sending units fall into the same pit. Mass production has allowable variations. Then add that some gauge manufactures have less concern about quality controls in both the gauge accuracy and the sending unit accuracy and then you get the market we have today. Unless you have very expensive, liquid fill, certified gauges, and sending units, I have serious doubts your gauge really has the ability to accurately determine a coolant temp difference of a couple degrees one way or the other. It may consistently give you the same numbers, but its anyone's guess if that number is dead accurate.

The car is either overheating or its not. Usually if its overheating, everyone knows. Its puking antifreeze, making funky noises, and everything is hot. Even if its not overheating, you may not like the numbers your gauge shows. That can usually be adjusted to some degree using various methods, but don't let your panties get into a bunch over a few degrees difference between two different thermostats of even the same brand. The purpose of a thermostat is to maintain a consistent running temp and eliminate the higher and lower temps, once the system has stabilized. If its doing that its functioning correctly. Then if running on the highway, or in stop and go traffic causes dramatically different experiences, fix them.