The thing people don't understand about Florida heat -- and especially the heat in Central Florida -- is that it's not the temperature or humidity that gets to you. It's not the heat on a particular day or week while you're visiting WDW.
As
@shh said above, it's the heat
late in the summer -- August and September especially. And it's not that it's hot and humid -- it's that you've been dealing with it since June and over time it just wears you down.
Living here is very different from visiting in this regard.
The temps may not be any higher in September, but you just get exhausted by day after day, week after week, and month after month of it. Onor near the coast, you have sea breezes and it's not bad. But inland, like Orlando, it can be brutal.