the pure, raw fact is that a class 2 bolt will tear up a class 3 nut.
Who told you this? The difference between class 3A and 2A is .0014" or less than a human hair. As per the Machinery's Handbook:
9/16"-18 NF 3A fit-Major diameter =.5625" max, .5538" min.
9/16"-18 NF 2A fit-Major diameter =.5611" max, .5524" min.
Manufacturing methods have improved a bit since that time as well and I think all they're trying to say is don't use cheap, poorly toleranced bolts.
