Hey, great catch :)
If you notice during the very first scene in the game, when he receives the letter from his grandmother, he mentions not seeing her very much after his grandfather died. He also mentions that they shared an awkward Thanksgiving (which sort of suggests last year's Thanksgiving).
So, given these clues, she probably returned from wherever she disappeared to about a year ago, which MC concludes is probably when she left Oakridge.
(in all honesty, I think I may have deleted a line where it was mentioned by Daisy that she left a year ago, so hopefully this explanation will suffice xD)