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Piston rings rotate during piston stroke, this is intentional and is caused by the honing on the walls. The ring gap will sometimes happen to be on the bottom of the piston when you shut it down, and more than usual oil will seep into the combustion chamber where it'll burn when you light the fire. Perfectly normal.
I don't have the site anymore but you can search and find a video made by an oil company where they do an infrared scan of a running engine to show the ring rotation - pretty cool.
My airplane engine (most airplane engines) are flat. We check compression in a different fashion than how it is done in a car, we have the piston fixed at TDC and run compressed air in through one of the two spark plug holes. Occasionally all three sets of rings will be aligned and a horrible compression reading comes up. Rotate the prop a few turns and try again - all is well!
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