I think (though may be wrong) that it is due to how the springer and pcp work, individually, like?
As I understand it, it would seem to go something like this:
In a springer, once the trigger is pulled, the spring starts to uncoil, and this shoots the piston on it's way up the compression cylinder, which continually compresses the air stored in the cylinder. About 2/3's of the way on the piston's way along the cylinder, the pellet starts to move along the barrel, having now got enough pressure behind it to start it moving . At this stage, the spring is still uncoiling and still piling