Why does the same product cost a different price at each chain?

Same product, same barcode — and a completely different price. The gap comes from each chain's pricing policy, operating costs, branch location and whichever promotions are active at that moment.

Discount chains price staples low to draw shoppers, but may be pricier on certain brands. Branches in expensive areas tend to cost more.

So there is no single "cheap chain" that always wins — the cheapest option changes product by product and place by place. Comparing each item is the only way to know for sure.