Top 25 rankings are based on a combination of talent and game results. The "talent" portion is calculated by looking at the skill ratings of players (excluding Work Ethic, Stamina, and Durability), with more weight given to a team's top 5 players. The "game results" portion is based on opponent rank, score margin, and result (win or loss) for each completed game.
Before the season starts, all weight is placed on talent - and then as the season progresses, more and more weight is placed on game results. By the end of the season, almost all of the weight is on game results.
There may be significant movement of teams early in the season because there are so few games, but as the season moves along, the best teams will rise to the top.