I would suppose...maybe an English chap would explain.
I recently bought a UK imported Browning Superposed that has screw in chokes...had a cylinder in the top and a modified in the bottom.
The gun has a barrel selective safety so selection played no part...Being that most people shoot the bottom barrel first on an O/Ur it kinda puzzled me a little.
What ever the fellow was shooting he was expecting his first shot to be longer...just the opposite of upland hunting.
The answer to this puzzlement are all in your post.
Unlike in the US style of hunting, the predominate wingshooting in the UK is driven birds. The Guns stand in a line, and a great distance away is a group of beaters (sometimes only with flags, or sometimes with spaniels) who walk towards the line of guns, driving the game towards you. Driven pheasants which are stocked at large estates, driven grouse which are native on the moors in the north, and some even shoot "flighted" mallards. And now, it has become popular to shoot wood pigeons over decoys or at dusk; while not a game bird, this is also often an incoming bird.
Our idea of pheasant, quail, and grouse hunting - walking hedge rows, forests, and fields and behind dogs - is called "rough shooting" or "walked up."
Because the overwhelming style of wingshooting is on incoming, driven game, having an OU with the bottom barrel choked tight makes a lot of sense.
