Tournaments, PVP1, War, testing decks for upcoming restricted tournaments, testing decks to share with starting players, PVP with friends, PVP with folks in the chat who are testing things as well, testing new cards...
Lots of reasons for this to happen. I imagine as players of the recent Mark Change tournament switched out their decks each game for the hours that the tournament spanned, you'd see lots of very weak looking, unupped decks in the Top50. Folks aren't grinding the AI all the time.