Apparently, both Skullgrin and Bomb-Burst were 11th hour additions to the line, replacing the Deluxe and Core class Energon Monsters that were originally going to be the gimmick figures of the Legacy line before they were cut and replaced by these two Pretenders.
Explains a lot, I guess somebody got cold feet over the Energon Monster, I wonder if we'll ever see the concept art for them?