Isn't there a good possibility that your sites with the old codes are now getting a fair amount of SE traffic, whereas your newer sites haven't had time to build up that quality traffic base?
Recently I was reviewing my stats with a reputable sponsor that offers both pay-per-signup and revshare. I noticed that my click-through and sales on the revshare program were about twice as good as on pay-per-signup, and something started smelling fishy.
But...I started promoting the revshare a couple of years ago, and only added the pps fairly recently. And, a lot of the revshare links are "upsells" from AVS sites. So, better traffic=better numbers. Kind of a no-brainer, but sometimes my brain takes the day off.
Most likely, though, Steve is shaving like a mo-fo. He's bad that way.
Disclaimer: The above was sarcasm.