They know that team A has its two starting guards returning this week, and will almost certainly play better than in their last 3 games. They also know that team B's dominant centre may have a bad ankle and may not play for 3 weeks. They know that two of team C's former stars will become eligible this weekend. They aren't yet sure if young team D is for real, even after 6 straight wins and 12-3, so they move them up, but slowly. Polls repeated every week also are a bit like Delphi Techniqe, with the summary of last week's voting tending to influence the decisions of individuals polled this week.
Results move toward more consensus as the season advances.
The computer calculating the RPI cannot know these things, or assimilate them in any meaningful way. It does not concern itself with whether games are Home or Away, whether team E played 5 games in 5 nights, whether team F is the defending champ, whether team G has won 10 straight. On the other hand the RPI is much more reliable, or consistent, simply because it is calculated. Doing what it does, it can easily give equal consideration to the record and schedule strength of every team. The experts being polled will know more about the strength of teams in their own area, but will have to rely on rumor and tips from other experts for their knowledge of teams in other areas.