It was Versus mode against an arbitrary opponent. I'm not sure that you can choose the format of the game. You can filter your decks by type (Theme, Standard, Expanded, Legacy) and the corresponding decks will show up but when you hit Play, I think you can be matched up against other types (including I guess Legacy where these banned cards are legal). Unless it's only when they can't by find an opponent with a matching type for you that this happens?