When your engine is running, you are measuring water temperature. The engine, when running, is always trying to heat up and the water in it is trying to keep it cool.
When you shut off the engine, the flow of water stops but the internal parts of the engine are still hot, especially the closer you get to the combustion chamber. The hot internal parts of the engine are now free to heat up the area where the temperature prob is located. This will cause a rise in LOCAL water temperature. When you re-start the engine, that heated water moves on past the area where the temperature prob is located and the circulating water will then re-cool that area.
The temperature fluxuation should be 15 - 20 degrees.