I could be mistaken, but I'm thinking that HS doesn't do full damage in the area of the entire targeting circle, does it? That could explain your second observation.
End of Aarojw's quote
I know that. The Hammer slam does x damage right where the hammer hits (or at least virtually hits) and y damage in the outer area (depending on the level). But that's not what I meant. If I assign the target to be dead center on the tower, the rook still might slam somewhere around the tower and not do full damage. But as I said, I am not sure if it does in fact not give full damage in such a case.