Many times, when authors are going to do a signing, they will sign an extra case of books, or more, so that after the signing, the store can sell autographed copies. However, in the event that a store aims to high and doesn't sell all of those copies in whatever time that store allots for author promos before they take their shelf space back, the signed books may very well be shipped to the B&N Warehouse, or to the distributor's warehouse, or even back to the publisher. And months / years / decades later, when some other store orders a hardback, a warehouse worker somewhere probably just grabbed the first copy they found and sent it off without realizing it.