Not entirely sure I'm understanding. The 2013 game has different decks than 2012. On top of that, if you want even more new content then you have the option of grabbing the expansions for additional decks. Typically they (the expansions) would also add extra single player content (new matches in single player, or new puzzles <3 ), though the later "expansions" for 2012 were new decks and nothing more. (Looking at the product description, looks like the 2013 expansion does indeed add new scenarios + challenges, etc and not just new decks.)
Just one last question, the expansion pack says it requires MTG 2013 so it is not an upgrade like a sports game since it seems to upgrade the 2013 version with even more decks?
So yeah. Think of the core games (2011, 2012, 2013) as the incremental sports titles, with the expansions just giving you more on top of that. Does that work as an answer? =S