I think APs were originally a little bonus for an Artist (in addition to the official, limited edition production run).. given to the Artist to keep, give to friends, or sell on.. and normally there would be a very limited number, maybe 10-15 APs (sometimes less) ..numbered in the same way as the production run (eg. 3/15 AP).. it was for this reason (ie. it suggested a link to the Artist) that some collectors used to pay more for an AP.. nowadays, not so much.
I am distinguishing here between contemporary (1970 - ) commercial Artist's Proof releases, and 'working proofs' ..not to mention historical definitions of an Artist's Proof.. all of which are slightly different.