Where are you measuring your trans temp?
Sump? Or the line going into the cooler?
Cooler-in will always be a lot hotter and vary more than sump temp.

The thing with converters is they have a slope to their slippage, they aren't on/off, so the slippage is constantly changing due to changes in input/output speed or torque input.

Even if the slip % is the same between 2 sets of operating conditions, if the torque (either input or output) is different, the heat generated is different.

And as the input temperature into a cooler increases, so does its efficiency. So you might add 20° going into a cooler from more slippage, but teh cooler might take 13° of it back out (7° increase on the gauge). But if you add only 10° maybe it only takes 5° out, a 5° increase on the gauge. (hypothetical #s to illustrate my point).

But the bottom line - if it works, it works, and there are then obviously more things going on than just common internet theory.