I have seen the same phenom when I was only shooting Glocks in volume. Shooting almost all 9mm then occasionally I'd break out my G22 for some tests scored. I have very frequently seen a small increase in time but a significant decrease in points dropped with the G22. I figured it was that effect you mention. Then Kevin B coached that exclusive 9mm shooting "may" lead one to "cheat their grip" some and that must be watched out for.
I also factor in shooting at "assessment speed" (call it .30-.35 sec) vs setting a PR in one's Bill Drill . . .