mass-hole

The reason you see the temp rise when you pull off and idle is that you like were generating a lot of BTU's traveling at highway speed but the air flow was also huge and was very adequately cooling the radiator, trans cooler, etc. When you suddenly pull off the highway, some of that heat is still finding its way from the cylinder walls to the coolant passages or oil. Or in the transmission, from the hot gears and what not. When you suddenly slow and lose all that cooling air flow, that heat that was still in the chunks of steel and iron that make up your engine/tranmission is still jumping to the fluids, but with less ability to shed the heat via the radiator/coolers/etc.

If you totally shut the engine off and leave it for a few minutes, you will see the coolant temp jumps. Same thing. The heat in the engine is still transferring and the coolant is no longer moving so it just continues to heat.

NC F150

Sounds like a logical explanation. So does Smokey's, and the electric fan theory.