That doesn't sound correct to me -- if you are definitely sure you don't need the 5th night, I'd cancel now just to be safe.
I'm not sure why there was a price difference at the GF...unless it's a slightly different view or something. Also, are you sure they were giving you the AP rates, or is there a chance you were quoted the current discount available to the general public (which was a few dollars more than the AP rate). Maybe that's the difference.
I'm actually quite surprised that there is any availability at these properties during the holiday weekend (Labor Day).