When ET and MPH Are Not a Match?

The formula is based on perfect input data, and the mph through the traps is calculated buy how quickly you go through the timing beams at the end of track. That is why the numbers are not perfect and some guys are higher and some lower.
You can see this if you get a few good runs with no tire spin and really close shift points, as a base line. Next get a couple runs on the same day but leave the starting line softer and use the same shift points.
You'll see a difference in mph, usually only 1 or 2 mph, but you'll know the horsepower has not changed.