I was checking out the brass today.  Part of the reason for better accuracy with the crimped load was the crimp doesn't expand to full chamber neck diameter.  There's still a little tension on the bullet if I insert one with my fingers.  Probably pretty important for this stubby bullet.  The case isn't being pinched by the throat either.  I trimmed them to max length, they're still about the same length.  Uncrimped max length cases blew out to the full neck diameter, maybe allowing the bullet to tilt a bit as it takes the rifling, or a bit before since the bullet is so short.  Anyway, it appears the crimp keeps the bullet straight for a greater amount of time.