Missing pieces are easily explained. They slip between sofa cushions, or pets chew or move them. It's a bit harder to explain how wrong pieces get in a puzzle box. I imagined a crew of library volunteers around a table, counting pieces to make sure the donated puzzles were complete. The table was crowded and two people sitting side by side couldn't keep their piles separate. Unwittingly, they exchanged a couple of pieces without altering the count.
Three extra pieces in each puzzle above. Both also have gaps, yet none of the spare pieces fit. I guess life is like that, sometimes.