I hope this doesn't look wrong when I type it, but what a "barrel or choke" is marked ain't always what it throws.
Patterns are more bore than choke, and pattern boards better identify what "constriction" one has than the markings.
Etched in granite somewhere ,someone chisled that a 20 bore was .615" one upon a time. The only absolute I can safely state about Shotguns, Chokes, and loads "There ain't no absolutes".
Granted from station to center stake is 21 yds in skeet, that does not mean that is the ONLY effective range of a skeet choke. Use extra hard shot, tweak with the loadings and the range extends. OTOH in the old days of fixed chokes only I can make a full choke throw a more effective, bigger and denser pattern with a full choke., than a skeet/IC choke bbl..that's what we did before screw in chokes. Give me soft shot and my recipe..anyone can make your skeet load look way to tight.
I always advise one to pattern at the range with the ammo needed for application and adjust accordingly. I tend to pattern a wee bit more distance for a cushion. Different brands differ, different recipes differ, same guns and chokes differ. Different lot #'s of shells/ powders differ.
IIRC Dave jut pattened a choke and found it Non- concentric, another member sent him one that was and patterns improved, I belive these two chokes were marked the same tho'.