I have no experience with PBS, but I see it this way:

Our scheduling availability is a cup, our schedule is the water. Management has a rough idea how big our cup is, but not an exact size. If our schedule (water) overflows we benefit through soft time.

In PBS, I imagine management knows exactly how big our cup is and will fill that cup right up to the brim, without a drop of water spilling.

If this is a bad analogy for PBS, I apologise.