IIRC the better quality ones like these Nendos are just factory rejects. GSC has a pretty high standard of quality, i.e. all faces with slightly misplaced tampo print go unused (which of course don't just get thrown away or destroyed as GSC would have it but used to make these instead).
Sadly not all bootlegs are made this way, other times the chinks just steal defective molds or worse, try to make their own copies from scratch, with varying degrees of "success"
http://archive.is/hx12g 
