After hearing mlb's explanation, I still don't get it. They say it's not a hr because it didn't bounce off the top of the wall.
Here's the relevant rule in the mlb rulebook:
(f) Any fair ball which, either before or after touching the ground, passes through or under a fence, or through or under a scoreboard, or through any opening in the fence or scoreboard, or through or under shrubbery, or vines on the fence, or which sticks in a fence or scoreboard, in which case the batter and the runners shall be entitled to two bases;
Maybe I'm missing something, but I don't see anything that says a ball bounding off the middle of the wall and over the fence is a ground rule double.