Though bending at the waist may change the draw length a little, the change in draw length is not enough to affect arrow placement at 20 yards and in. What bending at the waist does accomplish is maintaining proper alignment between bow and shooter so that whatever sighting/aiming system the shooter uses stays the same.
As I noted earlier, I still think the main reason for high hits is the fact that when viewed from above, the center of the deer, which is where we tend to aim on level ground, is just about in the center of the back, because from above, we can see part of the offside of the deer. Thus, when we shoot for center of the body from an elevated position, we're pretty much shooting at the top of the back. That's why it's important to aim lower.
And, as some have noted, deer sometimes do jump the string. That almost always results in a high hit or miss.
BTW, folks don't have the same problem shooting uphill, though the angle may be the same. Shooting uphill, they tend to shoot low. Perhaps that's because they can see more of the underside of the deer and less of the top. Visually, that moves the center of the deer down, accounting for the lower hits and misses.