Predicted stack time in Atlanta didn't happen.
Nice jetstream can bump airspeed by over 100mph (unlikely going S on east coast).
Airline needed the plane for something else, so authorized the pilot to burn some extra fuel.
First one is the most likely. Airlines also allocate a LOT more time for flights, since they get penalized more for delays now. Which means basically every flight lands "early". It's only about 750 miles at ~550mph. So 90minutes is plenty. Anything else is just airline schedule padding.
Edit: Quick look shows 2:20 for a bunch of flights. Shaving a hour off that is easily possible.
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