I reckon they don't offer the cheapest version of the same products online.
I've been shopping online with Sainsbury's and Ocado and I'm always disappointed by how little you get for the cost e.g - smaller packet sizes and fewer items.
I've just done a rare supermarket shop and my money went a lot further e.g I could buy a bag of JS own label broccoli florets for £1.50 whereas the cheapest similar available online always seems to be more than that. Or have prices started coming down?