I suspect the case. Starline brass advertises higher velocities because the cases are thicker and heavier. I've seen this when reproducing loads developed using one type of brass then switching to starline. As noted, most of the cases when sizing required similar effort. A very small number required what felt like less or more effort. This was one of those circumstances that probably required more effort to size.
In this case I was individually measuring the COLs for each round and noticed the anomaly. Had I been running these through my dillon for quantity, probably wouldn't have been caught and a subsequent FTF because the bullet was way over the recommended COL.
