I consider this to be non intuitive and an interface bug.
The statistics button on the deck list is pinned to the last opened deck. See if I click it.
Because I enter this from the deck list I would expect it to be about the entire list.
The fix would be for the statistics to be global by default. (Does it make sense to see your review statistics across all decks? It probably does.)
Then to see deck level stats you could either make a filter in this view or you could offer an entry point to stats from the deck view for a deck specific view.