The suggestion that ASDA is 20% cheaper is obvious nonsense as a routine state of affairs unless they were willing to make a 15% loss on the deal. Retail margins are approx 5%.
It all depends on what the basket includes and you may be sure that it involves almost every 2 for one and 3 for 2 offer they can muster on a given day that Tesco is not offering. Those offers are dragged out of suppliers with threats as to what will happen if they don't agree to fund it. Every supermarket chain does it; it's immoral.