I have seen this before on MM days, Ridemax tends to be pretty conservative and wants you to get on your other choices before they get a long line. Ridemax seems to think that the PP line is not going to change that much while you are on the other rides, since there is already a significant line. You should make the call when you get there. If you see a 20 minute line for PP when you arrive, you know it will only add 5 minutes when you ride it later. However, maybe there is a fluke and it is only a 10 minute wait. Go now!
It is all based on numbers, so on
average on a MM day, PP is already going to have a line over 20 minutes when you arrive in Fantasyland. Snow White and Pinocchio will probably get 10 minute lines later, so they tell you to hop on now and then do PP when the line is 5 minutes longer. It is all about collective time spent in line. So if it can lessen your wait for two lines and add 5 minutes to your PP wait, it will do that so your overall wait in lines in less. You of course, can decide to do it differently. But it is helpful to know, that if are in line at PP in the first 15 minutes or so, the line won't be too bad. Check that line in two hours and I will bet it is at an hour!