Hmm, *reading BGB* you have a point.
"..the rest of his forces move onto the table at the beginning of turn one"
Doesn't specify which table edge.
"The defender's reserves move on from his table edge"
So, the defender has his own table edge which implies that the attacker would also. And normally players have the opposite table edges.
So I would assume that that you roll for table edge as normal before the game starts and that is where either player's reserves enter from.
