I agree there are many factors, especially shoulder pressure. Shooting ELR I’ve noticed the same thing.
At 100 yards, it’s tough to produce .4-.5 mil shifts so I felt comfortable sharing it. Many active duty have reported similar shifts (in the .4-.5 mil range with that arca, or worse without it), coincidence or not. The more data we gather and share the better.
I wish I had taken pictures of the other targets, but another trend I’ve noticed is that vertical dispersion (group size) seems to increase slightly the closer to the tip of the forearm I set the bipod. It isn’t much, maybe .1+ mil on average vs. the bipod set in the middle or closer to the breech (all with the arca). I don’t have a large enough sample size to conclude that yet, and there are several factors, but it seemed to be a trend.