User blog comment:ACursedDoor/Grunkle Stan is not Dipper's future self!/@comment-5454762-20120903033535/@comment-27312415-20160324000238

I'm guessing it wa because of the trailer where it says that

"Who wrote the journals? Stan? Gideon? Old Man McGucket? These guys in robes? Dipper from the future? Mabel? Haha, it's not Mabel. Maybe it's Mabel...

https://www.youtube.com/watch?v=euvzXE24SrQ