It's pretty easy, yet a little difficult to understand. If you work at the castle, you get a room. Fuji, at his young age, works there, but Tezuka doesn't, he is still in training. When he will help his father with advising the king, he can move in there as well. But the kid of the tailor can live with his parents, since they live outside of the castle. No need to go to the dorm room.

Kids can be cute ne? Yesterday my 8, 5 and 2 year old nieces were here <3