The case
It was the line of 1994, printed on posters and T-shirts and repeated in every Forrest Gump impression. On the bench in Savannah, Forrest says his mama always said life WAS like a box of chocolates.
The marketing tagline used the present tense, and the impression circuit followed the tagline. The movie kept the past tense because Forrest is quoting his mother, who has already died by the time he tells the story.
Why our brains do this
The poster version was designed to be quoted, the film version was designed to be a character remembering. The quotable one wins.
Sources: Wikipedia — Forrest Gump · Wikipedia — False memory: the Mandela effect
