That Isayama (creator of Attack on Titan) has been planning for the anime to be a sequel to the manga rather than an adaptation of it.
There’s a lot of evidence to support this but the one piece that kind of kicked off the theory is the fact that Attack on Titan copied a lot from a game called Muv-Luv. The ending of Muv-Luv 2 is even nearly identical to the ending of Attack on Titan. In Muv-Luv 3, they introduced multiple timelines and it had a different ending. So if Isayama can copy and ending once he can probably do it twice.