The red marker at the left and center of this display shows the hypocenter of the blast. The T-shaped Aioi River bridge is said to be the aiming point for the bomb, which makes it rather inaccurately dropped. The zero-mile marker for Hiroshima city is much closer, being just to the right of, and on the same riverbank as, the bridge nearest the hypocenter. It's surprising that any structures survived, especially the large building at the bottom of the diorama.