[Bill Baer]

Racer 1320 makes a good point"Make sure water temp is exactly the same every pass, burnout duration is exactly the same every pass, launch RPM is exactly the same every pass, shift points are exactly the same every pass and you are putting the car into the beams at exactly the same spot every time."
It's possible that the motor was a littler colder on the last pass which would reduce it's tendency to detonate at the top of each gear and at the traps (You can't always hear when they are detonating) which would make the motor run better and pick up all the way down the track.
If you don't have a weather station to calculate density altitude I would recommend that you track and log the air temperature. It's a reasonably good indicator. If the temp drops 10 degrees, depending on your jeting, you could pick up as much as 0.1