A trace of burnt oil can leave a sooty or dirty plug.
I found this out with a Commando, where one oil seal had 'fallen off' the guide.
Usage was too small to be noticeable/measurable. And not noticeable to the rider.
If you have someone follow you though, they may observe a puff of blue smoke now and then.
If its on acceleration its rings, on deceleration its valve guides is the general guide.
It may improve with more running in though.
Have you given it a few BRIEF bursts of full acceleration to bed the rings in like the instructions say ?
Try swapping the plugs over too ...
