Slow 4 has slots for 12 players (ie 12 castles) but only 10 in rankings, how come?

If you're looking at the rankings through the Slow 4 game window, it only shows 10 places at a time (so if you're ranked 12th it will show ranks 3-12). If you want to see all 12 players go to Scoreboards, then Rankings and you should see the full list (the same goes for 40 player games, and 20 player double trouble games).

another possibility is that 2 players quit on turn 0, thus making them exit the game without harm to their level or the outcome of the game