Uneven Plug reading and similar piston wash?

I pulled my plugs the other day and noticed that the rear plug was a caramel colour while the front plug was very dark. I haven't done any plug chops, the readings are just from normal riding (I have brought it up to wot for a few bursts lately, whereas I usually don't).

Shouldn't the plugs be similar? The carbs are set the same and last time I checked they were synced properly.

I pulled the head to check piston wash. Would the difference in wash (seems small to me) correspond to the large difference in plug colour?


Extra info:
- I have noticed that there seems to be a bit of oil leakage on top of the had coming from around the rear dome. The front dome does not have this issue. Could that be causing the difference?

- when pulling my head, I noticed that the cylinder lifts slightly off the base gasket due to the weight of the bpipe. It only lifts about .5mm on the carb side. Should I replace this gasket? Could removing my head in the past (only done once before prior to this plug reading) introduced an airleak?


note: right side of images corresponds to rear cyl/plugs
 

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