Good question! In many card game booster packs, you got 15 cards: 11 common, 3 uncommon, and 1 rare. A box of boosters has 36 packs, so that's 36 rares, 108 uncommons, and 393 commons (enough to use as kindling in your fireplace

).
When they were going to release a super special card (gold printing, autograph, super rare, etc), they'd randomly replace one of the cards in a pack, sometimes guaranteeing one in a booster box, sometimes not. They aren't going to give you 16 cards, because weighing the packs could reveal the special ones. Most games replaced one of the booster pack uncommons with the special card. That's nice, because if you buy a booster box, you're almost guaranteed to get all of the uncommons, so losing one didn't hurt. But some games would replace the booster pack rare with the special card. So yay I got a special card, but now I have to collect another rare card to finish my collection.