That's true.
Skeet shooting is optimized at 21 yards. Pattern tests show an 18" circle with a hot core. People run 500 straight w/ 1/2oz loads.
As you know, sporting clays is a game played at greater distances to keep it from becoming skeet in the woods.
The level of precision to get consistent breaks after 30 yards is really impressive. It's hard. It takes practice.
And by and large the scores reflect it.
A choke set up to give clean breaks at 40 would be a bb gun at 10. Quite a handicap if chokes are fixed.
azgreg, I wouldn't get too hung up on UK choke descriptors.
The US tends to use points of constriction, and the UK seems to manage with fractions.
