If you look at the flight-aware track, what possibly could have killed his estimate was his (unplanned?) climb from 6,000 ft up to 11,000 ft. Your aircraft will burn the most fuel during takeoff and climb. In most aircraft, your climb speed is typically less than your cruising speed. Granted, fuel burn is less once you get to the higher altitude with a normally-aspirated engine, but so was his airspeed/ground-speed. I imagine winds aloft would have been somewhat higher at 11,000 ft too. I somehow doubt he planned for either of those when flight planning.

As to why he climbed, only he (and the controllers he might have been talking to) can say - maybe it was weather or just "Maybe I can conserve more fuel up here". The chosen altitude would have been incorrect for his direction of flight (even-thousand heading 180-359, odd-thousand 360(0)-179 degrees), which makes it more mysterious.