Actually, I suspect it's a smart move by Hornady. The 6.5 SAUM has been a wildcat for a while now, and as a result there are different versions of reamers, and I believe even GAP even offers two different throat lengths. As a result, it's something of a PITA to come up with a SAAMI acceptable chamber for an existing wildcat.
Hornady bypassed all that by using the RCM case, which doesn't have enough less capacity than the SAUM to make any practical difference in velocity.
