As I've said in a recent post on the Glass board, sometimes it seems as if the only thing that makes a piece of glass a 'second' is that it just didn't turn out exactly how the maker intended! So I guess if a weight wasn't near enough to the original it would be rejected as a 'second', even though it would look just fine to anyone else!
Of course, bubbles, pieces of 'frit', distortions in the dome, slipped canes or lampwork are all obvious faults. The ones where you can't see any reason for a piece to be rejected, I would guess are the ones that just don't come near enough to the original design!