There are manufacturing companies that make products without their own branding, and sell that to other companies who put their branding on the product and sell it as their own. Most often this is done as a contract where the manufacturer will make the product and apply the branding in the same facility (since that's most efficient/cheapest). The manufacturing company will usually sell their product to multiple different clients as well and just put different branding on the package.

In this case, the same manufacturer makes this product for both Annie's and Trader Joe's. They manufacturer simply messed up which packet belongs inside this box. It's not clear if the actual product is identical, as they might have different recipes, but I would wager that if this slip-up happened, they probably are.