To explain this in better detail, Bicycle Standard decks have an air cushion finish. This means that every playing card (on both the front and the back) will have little "pockets" in them that hold air (this is the case on every deck with an embossed finish). When a deck of cards with this quality is brand new, all of the "pockets" can hold air. This air allows for the smooth handling, even fans, spreads, and other moves to be done easily. The more you handle the deck (especially in a dirty environment or with unclean hands), more oils, greases, dirts, and other molecules will enter these "pockets", thereby preventing them from holding air. As this happens to a large space on the front or back of the card, cards will begin to clump and will not handle as smoothly.