They have the Sox much higher on that front (18%) than BP (10%) and Fangraphs (13.5%). I wonder how that's happening.
They each give 100% to make the playoffs (or near enough). They each give similar odds to win the division, so it's not about being in the coin flip game.
It's about their estimate of the Sox's strength. 538 rates the Sox as the best team going forward (essentially a 3 way tie with HOU/NYY), BP rates them as 4th (behind HOU/NYY, essentially tied with CLE), and Fangraphs rates them as 5th (meaningfully behind HOU/NYY/CLE, also behind LAD). You can see it in the number of wins each projects, 108, 105 and 106, respectively.
538's ELO is definitely relying more on current season results than the other two systems are - if not entirely on current season results at this point.