Sapphire makes more cards than ATI does, and supplements ATI's delivery system when ATI gets very large orders (at least this is a supposedly well-known situation), by having the correct decals, labels, manuals, and packaging on hand so they can ship direct to the custome for ATI to make up the balance of an order ATI doesn't have the cards on hand to fill. Also, it is reported that Sapphire is ATI's source for the example cards ("Reference" cards) that are distributed to ATI partners.
Yes, it is difficult to be sure which is which.