Choke tubes are slightly over bored on the chamber side. It was explained to me this was done so the wad never gets caught by the edge of the choke tube with dramatic results. So if a slight over bore segment causes tighter patterns using a more open choke tube would remove this effect. Perhaps you get tighter patterns from the tapering f he choke tube with a short parrel segment.
I think we over think chokes too much in all but extreme conditions. It's like shooters are thinking I missed that last shot because of my choke selection not because I did not put the pattern in the right spot. It's the Indian not the bow or arrows fault.
