Correct. The different spindles require different rotors or different inserts for the bearing races to allow using the correct bearing for that spindle.So I think that’s why they suggested a different rotor because the outer is a different size. All dimensions of the rotor still would match up with 84. So the inner seal and bearing remain the same but the outer changes, but finding the correct bearing to sit properly with enough room to tighten castle nut to spec with cotter pin in place has been the issue. I was basically told it’s an easy fix. But it’s become rocket science to me lol
Most people just install the bearing into the inserts/races within the rotor as it comes from the parts source & don't get into choosing what combo of internals they are going to use. That means using the correct bearing for each spindle type. One will require the SET34/A34 outer bearing w/appropriate rotor (metric spindle); one will need the SET3/A3 outer bearing w/appropriate rotor (78 spindle).