Brad, I'm not sure if I can say it right..but.. If the first gap is just a hair large, becuase you file it for worst case, you will have a small amount of leakage. You also have leakage around ring lads, behind the ring, and a small amount between the wall and ring. Nothing is 100%. So, if a small amoutngets by the 1st ring, you want to make sure it can find an easier path out then by putting pressure up on a ring that isnt designed to seal against that. Think of a damn and a lake. The spillways are much larger than what they will noramlly ever need to pass, because if you were to have a deluge, and limited time for the floodgates to react, you have to be able to pass all out thru one spot..the spillway. In the rings' case, you have a small amount of time to release the pressure bled thru to the second ring. (the piston's upstroke) So you want to make sure there is enough room to get it out. The 2nd ring wil leak all the same ways the top did, but you cant have any pressure between them. So you leave a bigger hole for it to escape.