When ET and MPH Are Not a Match?

There is a formula to determine what ET one should be running based on MPH in the quarter mile. ET = 1353/MPH or MPH = 1353/ET

my mph was 108 so it should run 12.52 but my best was 12.72
my mph was 104 so it should run 13.00 but my best was 13.29

When I punch my MPH into the formula, I find my ET is .2 or .3 slower than indicated by the math. I wonder where I am losing time? Thank you.

On the shift(s)?
PS: That formula doesn't work at all for my cars.