Heat flows from high to low like pressure. When it is in the radiator, the lower temp air absorbs heat from the cooling fins which in turn absorbs heat from water. Then when the water re-enters the block, the higher temp from the block is transferred to the water... The thermo stat not only regulates the temp, but the flow rate as well. How come you can take a "hot potato" from the oven and toss it from hand to hand without getting burned? It does take a certain amount of time for heat to flow, as we have all learned. How big is the opening in the thermostat housing, how about the thermostat itself? Without a thermostat some engines will pump water faster than heat can properly transfer and the motor will overheat.

What I am saying about flow rates and the t-stat, it goes from pinched up tight to wide open in a mater of 6-10 degree's. The resolution of the water temp gauge isn't that good. So when you are running pure water the t-stat might be open 20-30%. If you were to run pure antifreeze, the temp might raise a couple degree's opening the t-stat some more increase the flow rate and subsequently dropping the temp. This process will go on within a few degree's.

If you want to test to see if the motor gets hot, get an old t-stat, gut it and put in the plate so that the flow is restricted. Then drain the cooling system fill it with water find a big long hill and drive it up it and keep an eye on it. Then do the same thing with pure antifreeze. That way all you will be eliminating a variable.