It's elemental, actually. A PT is made for the tip to expand and the base is made to hold together and give deep penetration. An AB is made to explode. The plastic tip is driven down the center of the lead and forces it apart. It opens up much faster than a PT.
Mule Deer has written that the plastic tip generally doesn't drive down the middle, but rather veers off to the side. The expansion is driven by the big hollow point the tip rides in. My only recovered plastic tip, from of all things a Barnes 290gr ML bullet, bears this out. It was recovered under the shoulder blade a good ways from the bullet, which lodged in the spine between the shoulders (frontal shot from above). Of cours, stuff happens, and almost anything can happen once.
