Alternate Art cards are cards that exist with a different illustration from their base version. Some of them were unlocked during the Kickstarter and are exclusive to Beyond the Gates KS Edition. Any time those cards appear in KS booster packs, they'll appear as Alternate Art versions rather than the standard version.
These Alternate Art versions will not appear in the retail booster packs and are considered "exclusive" cards, meaning they can only be selected for Print on Demand once per year, making them highly collectible!