My [Jeff] Cirillo success isn't an oddity, but maybe unique. He signed two out of four cards, returning the other two unsigned (including my 2001 Fleer Tradition 'set' card). I noticed that there were two distinct dots on the unsigned cards. I don't think this was due to the fact that he stacked the cards together, causing the other cards to stain.
I've heard rumors about players marking cards that they return, but do not sign, to see if you would 'double up' them up with the same card and make another request. In recent months, I had read Florida Marlins reliever Todd Jones' column on Yahoo! Sports about the deal with autographs from a player's point of view. His column details how players would make little marks on the cards they return (unsigned) to see if you do end up making that multiple request.
I wonder if Cirillo took the time to dot the cards, so that he'd have a 'chuckle' in the remote instance I'd write to him with the same unsigned cards. Of course, it isn't like I have other Cirillo cards in my collection I could send him, but most likely I would have sent the unsigned 2001 Fleer Tradition again (since that is the card I really want signed).