In reply to BobHoliday:
I suspect people get unnecessarily hung up about this.
If you have one nominal 8.5mm rope and one nominal 8.3mm that's a difference of only 0.2mm (or 8 thousandths of an inch in old money); that is the thickness of 2 average hairs.
I don't know what manufacturing tolerances rope makers use but can't see it being in the region of +/- 0.01mm.
I would think softness & type/condition of weave of the sheath would have a greater effect than a small nominal difference in thickness.