"15 inches ?? I get about 8.7 inches."
Rough numbers only, 8.5 MOA at 200 yards is 17 inches (because MOA is an angle, it gets larger the further away you are looking.) So 15 inches out at 189y seems reasonable.
It's only coincidence that 1 MOA is really close to 1 inch at 100 yards.
A good exercise might be for you to calculate what the body of a deer might look like compared with your reticle at various distances.