Simply put, you are matching your line of sight with the arc of the bullets travel. That arc is going to be constant. If you compare to a 25 and 50 yard sight, the bullet arc meets the line of sigth at a higher angle at 25. This means that the bullet will rise higher above the line of sight and recross it sooner. With the 50 yard sight they cross farther away and the arc doesn't rise as far above the line of sigth.
For a graphic example do this. Fold a sheet of paper in half lengthwise. Fold it in half again. Unfold it and draw an arc (doesn't have to be exact) in the middle 2 quarters. Now take a second sheet. Make two marks on the top edge, about 1 1/2 and 3 inches from the left end. Now, put the left corner of that sheet about 1/2inch above the left end of the arc and pivot the paper on this point. Line up the first mark (represents 25 yard zero) on the arc and make a mark wher the arc recrosses the edge of the second sheet. Measure how high the arc is above the edge of the second sheet at its highest point. Now do the same thing with the second arc, the second mark represent the 50 yard zero.
Even though the arc is constant, those intersection points change percetibly and the distance between the arc (bullet path) and the edge of the paper (line of sight) change considerably. Your paper was covering the bottom end of the arc, but it changed similarly. This is in no way accurate to the results you will see with the actual thing, but it gives you the idea.