They are either. 0001 under or .0001 over their rated size. So a .089- gauge pin is actually sized .0889 and a .089+ is actually. 0891... I believe that's what they mean by "Go" or "No Go" pins as well. The Go pins are slightly under and the No Go are slightly over.
In all honesty, testing and classifying wheel bores to a .0005 +/- tolerance is probably bordering on the ridiculous... I mean when you think about it we're talking about pieces of wood, plastic wheels, and nails. But that being said, It's the ridiculous level that we've all taken this to that makes it so much fun.