You are correct, these cards are not in booster packs. For the past few years, Wizards has put out 30 card sample decks to introduce new players into the game. This year they decided to put into these packs a number of reprinted cards not actually present in M15: see here for decklists. Like the article mentions, the Deck Builder's Toolkit for M15 also has the extra cards.

Since putting two of these sample decks together gives a basic 60 card, two-color deck, Wizards expects a number of people to do just that to build their first standard deck. They made those extra cards explicitly standard legal to allow people that option; most people would assume by default that such a deck is playable.

Mystery cards in Magic 2015 card list
How long does it take for backlinks &amp; linkjuice to appear in your link profile and have an effect?
