I have no idea what the piston on the left is. There could be a lot of reasons for the slight differences in sizes though including slight differences in manufacturing. Forge vs cast etc. Looks like someone just made something they had work in a pinch after a seizure. I've seen some wildly mismatched bores and piston on some old 350's and 400's that ran just fine. I even pulled apart an old H1 triple that had 3 different size bores. You just never know. People will sometimes just keep seizing one side without ever really getting to the source of the problem (often an air leak) and keep boring that side larger.
With that crank rebuilt it looks like you're on your way to a smooth running ride. Keep em coming.