What is being described by these two terms is that this is how much of the light (getting by the secondary obstruction) that is getting reflected onto that spot of the image plane. If you put the secondary obstruction to the incoming light into the calculation, you would never get 100% but that's something else. The 75% zone is where only 75% of the incoming light gets to the image plane, the rest never getting in the tube, not hitting the primary or the secondary on it's way to the image plane.