Because large grocery store chains are not in retail - they are in finance.
They make a fortune with the cash they get at the tills (net-0) before they pay it to their suppliers (net-60, net-90).
You need to annualize that 3% to make it comparable. A grocery store isn’t buying everything at the start of the year and selling it at the end. If they do 12 inventory turnovers in a year, that 3% margin becomes a 36% return for the same amount of capital.