I'm just going to through out a crazy theory here:
What if Grunkle Stan is a version of Dipper from the future? At some point in his original timeline, he came to Gravity Falls, and started investigating. Along the way, he wrote the Finger Books. Eventually something disastrous happened to him, likely caused by Lil' Gideon, that turned him into the Stan he is now. Being from an alternate timeline, he doesn't have any of this Dippers memories after the point where their histories diverged.
He had the opportunity to travel back in time, and decided to take it in order to make a better future for himself. Using his birthmark he was able to convince his parents of his identity, and the need to send Dipper & Mabel to Gravity Falls so he could change the outcome of his life.
Stan is almost always wearing a hat, just like Dipper. And I may be wrong, but I can't think of a time where its really shown his forehead, he has his hair covering it.