This isn`t seen on a gauge but with a laser temp reader. Okay. Think of it this way. The coolant in the engine gets up to temp, the T-stat opens at that temp, the hot coolant goes into the radiator.
The coolant in the radiator, which is considerably cooler, is being circulated into the engine, when the cooled coolant reaches the T-stat it closes it, the engine builds back up to temp which kicks open the T-stat and the process starts all over again. That`s why you keep seeing the temp go up and down as the "cooled" coolant from the radiator circulates in the engine cooling it down only to build it back up again so the process can start over. Something else many don`t understand is when they get a mechanical temp gauge is why the temp rises when the engine is shut off, it gets higher because the coolant is no longer circulating.