Combustion chamber carbon build-up : an odd pattern?

[dtype38]

Due to the demise of a thrust bearing I had to change the crank on my race engine this week. It's a full strip-down job as the head has to come off to get the timing chain off the crank. No biggy, but when I looked at the chambers and pistons the carbon pattern wasn't anything I'd seen before. Now I've run this engine for years and it likes to run rich (like 5% CO rich - its just the way it wants it!) so when I take the head off I usually expect to find 6 black cambers and 6 black pistons. But last time I rebuilt it I put some different pistons in and this time they weren't all black. The chambers are, but I've never seen completely clean sections of piston before. Anybody know if its something I should be concerned about?