A bullets trajectory will cross the sight plane in two spots. If I comprehend your post correctly these two spots are ~20 yards and ~100yards and you are refereeing to mid-range from 20-100. Since the bullet actually travels in a rainbow type trajectory the only explanation for low shots between 20 and 100 will be your sight picture. The bullet isnt going to bow upwards as it travels through its flight pattern and your grip should be the same rather you are shooting at 20, 30, 80 etc.
The only thing I can figure is the allusion of your target in reference to your sights are not how they appear. Light conditions can affect this as well as the distance between your eyes and the sights and also the distance your eyes are above the gun. For example, do you ever bend your neck to look down your sights or do you raise your gun up to eye level and are you consistent in doing so? The same goes with how far you hold the gun in front of you
do you shoot with locked elbows sometimes and sometimes do you bend your elbows
With a Point of Aim at 20 and 100 yards you should be shooting high in between those distances and not low.
Be Safe,
Scott
