Appreciate you doing this I plan to do a similar test though not as thorough as yours, I have lee collet dies as well as rcbs and redding fl dies. Bart b has got me thinking as well. Runout is always lower for me when using the lee, but Bart has pointed out neck centering on the shoulder is better with fl dies. I have a McMillan stock coming for my 06 , it has a hart barrel and is as close to a highly tuned rifle as I'm going to get, so no excuses other then, me weather and my loads.
I noticed the one flyer kept you second group from being equal to the fl group, and the group had more rounds touching.. Hate to ask, but how did that one shot feel? Possible it was wind?