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My scales have the same 3 settings as you. I only ever weigh myself in stones and lbs. But my scales never give me the same weight more than once. If I were to go on and off them 3 times they would give me a different answer each time. Sometimes the difference between the highest weight and the lowest weight can be as much as 4lbs.
So I think comparing between stone and lbs and any of the other 2 settings would just give me a massive headache to be honest!