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This thought process may seem odd, but makes sense to me.
My planer instructions allows for planing material 3/16" and thicker. Since you can not lower the cutter below the lowest setting on the repeat-a-cut, the 1/8" setting was needed (they are indexed in 1/8" increments). When they changed the minimum board thickness to 5/16" they left the repeat-a-cut the same.
So it could be possible that the first model that they made required an 1/8" setting and they just never changed it. If you need to plane boards thinner than 5/16" there are methods available.