I think the main reason is simply geography. Unlike CFB and NFL, high school sports allow for away fans to travel easier and together, creating a more balanced volume level from each side of the bleachers. Of course, the bleacher size for the away teams will typically be smaller than the home's, but if it is a big game between local schools you know the fence will be lined as well. I think during the regular season the team's talent disparity will typically outweigh the fan's influence of the game.
That being said, .25 points is quite low, but I think 2.5 points would be too many.
PS: I really enjoy using your Predictor Tool.