The answer is most likely strength of schedule.
This is a significant factor in the calculation. If your team has a low overall strength of schedule and beats a team with a higher rank and a strong overall schedule strength your team will benefit in the rankings, but it does not guarantee your team will move ahead of that higher ranked team due to your low overall strength of schedule. Each situation is different and results may vary.
To find out more about how the rankings work read this article: How do the MaxPreps computer rankings work?