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.
Which goes against what has been said here: https://youtu.be/YOKqAIMh-iA?t=1538 I'll admit both answers can be correct (retailers send the prices for instacart to baseline off of). They have historically marked up the product with their better grocers (who in turn give them a %3 discount to offset credit card processing).
My understanding is that the prices are set by Instacart, based on estimates on what they've seen the store charge in the past, plus some markup for price change risk, and probably some amount of profit/cost of maintaining the pricing data and product catalog.
