Gauge Temp Lower Than Radiator Temp..??

Not for sure why I am getting this variation, but when I let the car idle for 20 min., the aftermarket (i.e. sunpro) mechanical temp. gauge tops at about 170. Now , I have the cap off the radiator with a good thermometer placed in the coolant. The meat thermometer reads 189, the temp gauge reads 170.
I don't know why I have such a variation. The gauge is new (not that it can't be bad out of the box). Plenty of flow. 180 stat is new.
Now, if I shut her down, the temp gauge will move up to a hotter reading ( which the engine would because it's already warm and there is no flow at that immediate time). But I am wondering why they are not alot closer in readings while it's running.
Sensor is in the port right next to the radiator hose inlet on the intake manifold.
318 motor, headers, older SP2 intake, eddy 1406 carb, MP dizzy.
It's not the end of the world, but would like to know why I am having the difference in readings.
As always, any help would be appreciated. Thanks in advance.