1) muscle memory-- your mechanics degrade less over the course of 200 rounds fired than they do over 500, so you're able to maintain a standard throughout more of your total practice time. You're also having fewer days in between, which reduces erosion.
2) more range sessions = more first shots of the day. When draw a firearm for SD purposes, you're probably doing so from a cold start, rather than getting into an altercation in the parking lot of your range. Higher frequency gets you used to starting cold.
It also stands to reason that you're never going to have or use 500 rounds during an SD situation.
Currently Own: Beretta PX4 9mm, Glock 23 (Gen 4), Glock 19 (Gen 4) x2