Ah; The old bug-a-boo of shot string rears its ugly head again. realising that most all shells used today have some form of shot collar the following will ignore that. 3/4 oz of shot in a nominal .550" 28 gauge bore will have a column length of about .910".
3/4 oz in a nominal .615" 20 gauge bore has column length of about .730" while 7/8 oz would stretch out to about .850".
To the best of my knowledge it has never been proven that this has much (if any) bearing on the actual length of the shot string in the air.
i believe you can likely rule out a shorter shot string as being the basis of your Improved shooting.
In fact I can think of no ballistic reason how reducing the shot load by 15% can result ion a more effective load.
My best guess is that due to the lighter recoil you are just "Shooting it Better".
