In other words, close blast radii reinfoce each other, creating more boom than dropping separate bombs!
I found this out a long long time back with a Ju88.
I was having fun pickling a single 50kg bomb at an old strat. I think it was a city. I would drop a single 50kg on a single building, and it didn't go down. I came around and repeated. It was still up. A third finally finished it off. However, choosing another building I dropped 2 in salvo, and together they both were able to take down the building in 1 drop.