I could possibly be a combination of the 2. But given the fact how smoothly the first two rounds were seated, and also the fact that I measured the shoulder jump and neck tension on all cases (almost identical), I find the chances of only one case being too large, as a leading cause, slim. And if the larger case was the root cause, not sure how it explains the excessive seating depth I got.
At this time, I am more leaning towards an issue with how I places the case 'on' shellholder vs 'in' shellholder. If number 2 (case sizing) is also an issue, then I am sure I will see the same issue reappear in some form when I seat the remaining cases. Will update this thread, once done, so we all understand the root cause here. I am going to put this up on top of a shelf as a reminder of my foolishness.