Dealers play by a different set of black jack rules than the players. An Ace in a dealer's hand is always counted as 11, if possible without the dealer going over.
I can't figure out if the dealer knows his hidden card or not? Of course, when dealer has 10 or Ace showing, he checks if he's got a blackjack.
Blackjack expert Henry Tamburin tells why you should hit your 12 against a dealer's 2, frustrating hands in blackjack is being dealt a 12 when the dealer shows a 2 upcard. You could also be dealt a soft 12, which happens to be Ace - Ace.