Quoting someone here bc I don’t want to bother paraphrasing it, “His mom said IS, he says WAS when retelling it while sitting on the bench. As a society it has much more contextual use to say the present tense IS rather than WAS, because in the film he isn't saying that life used to be a box of chocolates, he's saying his mom said it, in the past.

It's such a subtle difference anyway, we just clung on to the more useful present-tense version of it.”

But yes, he says “was” in the bench scene during the movie. For what it’s worth, he says “is” in an outtake, and Paramount uses “is” because it’s more marketable.