As others have said, the substitutions can be quite amusing. In fact, if something goes out of stock between your first making the order and then deciding to add to it, you are shown 'suggested' alternatives (on Tesco's site, anyway).
A few weeks back, I had initially put flour in my trolley. A few days later it was out of stock and the suggested substitution was a packet of lemon cake mix. Of all the thousands of things that I might have wanted to make with the flour, why did it pick that one? Of course, if I had actually been planning to use it for a lemon cake, I would have wondered how on earth they had known, and if my tablet was bugged! 