What changed? Is it a different team? The rankings are based, at least partly, on comparisons to other teams.
When Alabama played Texas A&M, it was the 4th game of the season. After the 4th game, Texas A&M was 2-2, having played both Alabama and Clemson. They finished 9-4. When would the sports people who vote in the AP poll have a better idea of the quality of team Texas A&M actually is? Before the 4th game or after they played 13 games.
The final poll is the most accurate. That is what the team will be known for in the records. Alabama was ranked #1 in 14 out of 15 polls. But in the end, we will be recorded as being #2 for the 2018 season.
But, like I said, if you want to insist on using the rankings when we played, that makes Alabama look better. By my way of it, Alabama beat the #4 team, the #6 team and the #7 team. By your way, we beat 3 teams ranked #4. You are arguing to make Bama look better.
Also, did the 14 teams we beat lose to the #1 team? Or did they lose to the team that finished #2?