The skeletron promo pack has 7 cards, but its comprised of two "pools", skeletron pool + dungeon pool, where skeletron pool only contains 1 card, just itself. There's a 100% chance that one card from the first pool will appear in the pack, and the remaining two cards are then random between all pools, which is 1/2 because it first rolls the pool, then the card, and since skeletron is only 1 card, it will be skeletron with 1/2 chance. Result is that you get 1 or 3 skeletrons 25% of the time, and 2 skeletrons 50% of the time. Then roll the 1/15 foil chance 3 times. If any card is foil, all copies of it appear as foil in the pack. I suppose the last fact is the surprising one and skews the perceived chance