my theory on this "secret" price savings.

Instacart asks if an item is out of stock if it ok to replace with similar items (might even default to this). Instacart has negotiated deals with product manufacturers directly. Instacart replaces "out-of-stock" products with the discounted products. The manufacturer issues coupons that can be used at the till. Instacart takes most of the savings themselves but also shares a little with the consumer. Win-win-win.

2nd Theory. Apoorva has developed the largest coupon clipping operation to date. Puts "extreme couponing" TV show to shame.