Bill Cipher, a mind demon, invented false "AI" that could generate art after being trained using stolen material. His weird relation with time led to some of his AI "art" to leak into the past.
The "AI" art often got details wrong, such as hands with too many fingers. Human artists also found hands hard to draw, but it was a tell that AI made it.
When two twin boys were born to the Pines family, they got the names Stanley and Stanford. Because of Bill and the "AI", Stanford was born with six fingers on both of his hands.