I don't know if this would help you or not, but I shoot most on my handguns with open sights. I hardly every shoot off a bench while shooting open sighted handgun. About 3 years ago I was shooting my new 510 GNR single action handgun, getting ready for my up coming black bear hunt. I was shooting it off hand, and at 25 yards I was getting very good groups. I wanted to see how tight I could get the groups, so I went to the bench. I started shooting and my groups were great, but there were all a lot higher than when I shot off hand. The only thing I could concluded is how I was holding my head, as I was looking down the sights. As soon as I got up and shot the gun off hand, my groups went right back to point of aim. My buddy was with me and thought it was odd as well
Give that a try, as that seemed to be my problem.