It looks like you're new here. If you want to get involved, click one of these buttons!
Obviously, collectors like to have some idea as to how rare a poster is, particularly with high value items. They use auction archives as a guide but its often difficult to tell whether the same poster is being relisted/resold or a different one is being auctioned. Condition reports and photos don't always help because the item is usually re photographed and condition is subjective depending on who is grading it at the time.
Sometimes, collectors will look at auction archives and see that a particular poster has turned up say, 5 times but that doesn't mean there are 5 copies. I know of one case at least where a very rare poster was sold in fair condition with quite a bit of damage. I happen to know that the same poster turned up years later in a major auction after being linen backed. The same poster was later offered for sale again. Collectors searching through auction records might assume that a few copies exist when it was actually the same poster being resold.
This is not a criticism in any way of the major auctions but I just want to point out that even though a poster might be appearing more than once in a major auction, it doesn't necessarily mean that there are multiple copies. It could well be the same poster being resold.