The case
You never eat one Cheez-It, so the brain pluralizes the brand. The box has always been singular, one of those names that reads like a command rather than a description.
The same instinct hits a lot of snack names. We say "Doritos" and "Cheetos" because those really are plural; Cheez-It isn't, and memory files it with its neighbors.
Why our brains do this
Snack names are mostly plural, so the singular one gets regularized to match the shelf around it.
Sources: Wikipedia — Cheez-It · Wikipedia — False memory: the Mandela effect
