I'd expect the prorating to spread the dates evenly across the decade precisely to avoid this kind of periodicity. It seems to suggest dates have been inaccurat

I'd expect the prorating to spread the dates evenly across the decade precisely to avoid this kind of periodicity. It seems to suggest dates have been inaccurately recorded with year precision as, say 1690 when some author along the line didn't know if it was 1686 or 1694.