People missing high are just half the reason the backstop needs to be 20-25 feet high. The other half is people missing low, and by low I mean short of the berm such that the round skips. Technically a ricochet, they're much more likely than a round inadvertently being shot 20 feet high.
The ground in front of our 100 yard backstop is testimony to dozens of rounds impacting short, as much as 30 yards short, and the divot pretty much proves the rounds skipped off the ground to go high up on the backstop. Surprisingly, the 50, 300 and 600 yard ranges have very few such divots.