What do you think happened?

When you floor it, the cable pulls on the throttle. When the throttle stops at WOT, and the gas pedal is not yet on the floor ; then if the anchor-clamp is just loose enough, the cable can slide forward. You might have/ should have, felt that .
However, with the cable slid FORWARD, it would now park with slack in it! If this happens to you, then your cable is the wrong length for your geometry, or the other way around.
But this has nothing to do with the changing idle speed. This would require the cable to move backwards in the clamp towards the firewall, or the carb to move towards the rad away from the anchor, neither of which is a realistic possibility. So IDK what happened.
About the only things I can think of is that the cable is not running back out of the sheath. This would make your pedal sit lower than usual, or the pedal weld is separating, or like mentioned, the simplest of all is that the carpeting or something is bunched up on the cable inside the cabin.