I'm no expert, only a lurker. I just signed up to to add the experience I had over the last two days.
But, I think that if the top hose that leads to the overflow tank is not clear of fluid, the engine sucks a percentage of the coolant directly from the overflow tank *before* it passes through the radiator.
This means the overall temperature of the fluid is higher, but not enough to be overheating the engine because most of the fluid is still passing through the radiator.
Only a theory and I would hope that someone with more knowledge could set me right. But it did solve my problem.
