When I hunt grouse I use #7 1/2 shot and I doubt anyone in His right mind would use #10 shot.

But this was a different time; I was trying to find out if anyone would know why He used that particular load. He must have thought it had some advantage.
I don't think it would fill the bird with as much shot as some say, there are only 5/8th of an ounce. It was shot through a cylinder bore with 3 drams of powder, so that it would open fairly wide.
Reading the book you see that the author had a very strong sense of sportsmanship.
I have to think there was more to it than it seems.
Pete