Its all done by people following orders from a computer. Every item has a barcode consisting of numbers, and the computer comes up with alternatives (based on numbers). Most of the pickers work at nights and I should think most are already stressed out, as we are, and very tired. (Yes a lot work through the day too and get in everyone's way).
I think they are just not concentrating or they'd notice the absurd substitutions suggested by the hand held computers they have, and use a bit of common sense. I once worked on the tills at a supermarket and the staff are not all morons. The pickers used to share the hugely daft substitutions with the rest of us in the canteen.
Most alternatives are reasonably sensible.