They don't need to explain anything. Most of the time the founders and investors are in on it together. Just get an exit.
EDIT: The discounts come from the cost savings stores will see from delivery companies, to clarify to people commenting margins are not that high in grocery business.
1) These delivery services are become sales and marketing channel for stores.
2) Cut down time it takes for cash register scanning each product and bagging them, pick up guys do that for stores.
3) Bulk pickups-- e.g., if I can have delivery company gather milk directly from back store, my store staff does not have to spend time replenishing the shelves.
> "The average profit margin for grocery stores is 1.3 percent" says Jeff Cohen, a grocery industry analyst with IBIS World.
-- http://www.marketplace.org/topics/business/groceries-low-mar...
[1] http://mixergy.com/interviews/brick-mortar-retailers-herb-so...
Net income after labor, distribution, and fixed costs ranges 1-6%, depending largely on market share and fixed cost control
Reference: I helped traditional retailers with price optimization for almost 20 years