It's likely just there "because it is". I can't see any reason that it would be better than a standard lap joint. Any pressure applied would be opposed by the other side of the ring gap, so while pushing one side out and down, it would be forcing the other side of the gap in the opposite direction, in and up, creating a bigger leak between the cylinder wall and compression ring.
I'm not saying that actually happens, but it does wreck the theory that the rings are pushing out because of the angle.
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