The RPI (rating percentage index) is a standardized formula based on winning percentages, and the Top 25 rankings are a simulated media poll.
The RPI (Rating Percentage Index) is a measure of strength of schedule, and how a team does against that schedule. It does not consider margin of victory. It is used by the WCAA as one of the factors in deciding which teams to invite to the National Tournament and where to seed them.
The basic formula is 25% team winning percentage (WP), 50% opponents' average winning percentage (OWP), and 25% opponents' opponents' average winning percentage (OOWP). When calculating winning percentage (WP), the RPI weights a road win as 1.4 x a normal win, a home loss as 1.4 x a normal loss, a road loss as 0.6 x a normal loss and a home win as 0.6 x a normal win - neutral site games are not adjusted (weighted at 1.0).
Top 25 Rankings
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.
More information about RPI can be found here. More information about the Top 25 rankings can be found here.