I’m not too sure of it myself, anon, but I could try.
You see, with John Hurt being the secret/hidden Doctor, he presumably was the one who ended the time war, killing all the Time Lords. He presumably came after 8, making him the actual 9. That makes Christopher, who was 9, now be the 10th Doctor, David/10 being 11 and so on.
It’s all really confusing, but I hope I helped!