The explanation I heard was that it was likely Mary and Peter hallucinated Jesus only a few days after he died. That’s a very common timeframe for when people hallucinate seeing dead loved ones, and the early descriptions in Bible match the flavor of dead loved-one hallucinations people typically have, with the figure assuring the person everything will be all right and whatnot. Other descriptions (like Jesus appearing to all twelve disciples or crowds of people) seem to have been written later more as persuasive arguments, with doubting Tomas acting as the stand-in for the skeptical listener. This is all from “How Jesus Became God” and I have no idea how mainstream or fringe the author’s views are.