With the defending king on the edge away from the corners, the result depends on the bishop's diagonal. When the bishop cannot reach the long diagonal that covers the interposition squares, the rook cuts it off and wins.
The defending king does not always get to the right corner. Here it is on d8 with White's king on d6 in opposition, and the bishop on g2 is on the "short" side of the position: it has only the h3-c8 and h1-a8 diagonals available, and White's rook can interfere with both.