I worked as a picker for Asda when I was a student, and there are different people pick different areas, so you may have had four of five people pick your total order, frozen, chilled, fruit n veg ambient etc.
You get marked down if you do not substitute with SOMETHING.
Our manager pulled us in for reviews once a month to discuss things like pick rate and substitutes, and at our store you were not allowed to 'nil pick' you had to substitute something, even if it was random, as you may make a sale....which is hilarious because really it ends up creating more work as then you end up with returns
Many of the pickers are school leavers or uni students on min wage, so don't really have the experience to know what may be a good substitute, really they don't actually care they are on minimum wage and are not planning to make a career out of t his, they just want the managers off their back. Also pickers start work at 4 or 5am so are exhausted as is quite physically demanding work and then are probably going straight to lectures etc
Now I order from asda home delivery service and I thin its a great service, you pay a few quid for someone to walk around the shp for you, pick pack and deliver. I love it, and if the substitutes bad you can send it back you are under NO obligation to keep it - however often the substitutes are better and higher value but you pay the same price