Embassy is one of those cards that has the potential to buy four Provinces by turn 12 with very little support. Opening Silver/Silver with the purpose to acquire an Embassy on turn 3-4 is pretty solid.
Embassy is a swingy card. If it turns up early in the shuffles look out, fast game. Obviously if it shows up late in the deck throughout the match, not so much. Going double Embassy is even more swingy, because if you catch both Embassies and force a reshuffle during the critical VP stage you are pretty much cooked, but if one Embassy appears on the opening hand after a new shuffle often then it is gravy.
Looking at this board, again there appears the Hoard/Apprentice, which is a solid combo but will probably lose more often if both players attack the Province pile. There are no buys, which probably hurts the Apprentice player more, as he can not double-Duchy to try to force a longer game. I do not know for sure, but I suspect that if both decks perform up to average the Emb/BM deck is stronger.
Oh, also, note the Swindler, another swingy card which helps the non-terminal Apprentice deck and does not fit well into the Embassy deck. It may be enough to squeeze out the difference.