I think the reason the author used such a small charge of very small shot, with a relatively heavy charge of black powder, in a cylinder bore gun, was to have a pattern that opened very quickly at short range.
I'll second that. Seen thin logic in action many times. A person has a problem with the gun shooting too tight. He drops the shot charge, to widen the pattern by the relative overcharge of powder. It works, but there are too few holes in the target. Then the person takes a smaller shot size to increase the pellet nomber, figuiring that at the close range the load is intended for, the smaller pellet will have enough energy to do the job. And it works, too. Possibly not the best way to solve the bulletlike-pattern problem, but works. I've seen hares shot very dead with a load of 20 gramms of #7 over a powder charge normally loaded for 35 gramms, in 12 gauge, at ranges from 5 to 20 meters.
