Do you think it’s possible that the price was forced to stay the same, and the difference is made up by using cheaper and lower quality food? You’re assuming the quality and size of these lunches has remained the same and I have seen no evidence that suggests that’s true.
They’re limited by budget, and need to attempt to put together a menu for hundreds (if not thousands) of children with different, legitimate, nutritional needs. Every single day. It’s an impossible task given the paltry amount they have to work with. The only way they could possibly do it while keeping the funding the same is by cheaping out on the quality of the ingredients.