From what I read its basically due to the difference of style in leagues. In Spain's league players box out more on rebounds. In the US players tend to jump up and try to grab it more.
http://nypost.com/2015/11/12/ex-coachs-inside-look-at-frail-porzingis-transformation/
Also to add to that he's playing with arguably the best box out Center in the league with RoLo who gets him even more space. Hell LMA became a better Rebounder too when he was playing with him as well. You could make an argument RoLo helped LMA overall as well by doing the grunt work that he is still doing next to KP.
Also if like yesterday they are doing a small ball lineup he's gonna have that advantage in height and be next to Melo who is arguably the best stretch 4 in the league. That is going to get defenders out the paint and gives KP even 1 on 1 opportunities to rebound. That's why the rebounds stats tend to be offensive RB's more than Defensive RB's.
Like I can't say this enough as much as KP's fast development has helped the knicks win games, the roster (and staff) of the knicks have helped KP develop faster. I don't think he could do what he is doing as quickly if he had been drafted by the Lakers or Philly.
Did Phil plan it this way idk, but if he did wow