From my understanding, the prices are set by the store, which can be corroborated by the DoorDash/Uber Eats models. It's impossible for Instacart to handle the prices for all these stores, so they unload that responsibility to the individual stores. The store probably forgets to drop the price, or will mark up volatile prices so they won't have to do weekly updates. I can bet you that you can also find some items that are under priced.