That's out of date. Generally speaking pillars won't appear on the slot machine for a normal deck, but decks that don't have a sufficient number of unique cards will have some pillars inserted. This was put in to nerf farming decks; otherwise you could create a deck with 1 rare and 29 pillars, and anyone who beat you (which would be pretty much a guaranteed win) would automatically get 3 copies of that rare since it would be the only thing on the slots--so it adds pillars instead so you'll mostly get those.
