The auto fill cells are to assist you by giving you a realistic projection when it comes to Passing and kicking percentages.
Total automatically uses the current scoring system to give an estimate of the projections entered. The average divides by 17 regular season games.
Some fields have been simplified to help with projecting players, and the total calculation on each sheet augments the scoring system to help with that.
For example the field asks for 'Total Tackles' when the scoring system actually separates between Solo, and Assisted. The calculation towards the 'Total' is augmented to give the best estimation.
Fumble recoveries and such are eliminated from the projections, as they are far more random than other stats, even interceptions. This is to help not over complicate your projections.