How do you fraudulently overcharge in a grocery? They see the price on the shelf, and I'm guessing it wasn't something as simple as it scanning in as a higher price. If they don't like the price they can not buy it.
In short: the packaging might say there was say 1lb of chicken tenders. In fact, there was only .8 lb.
It looks like this applied to deli/meat items? So not pre-packaged foods on the shelf, but the store-packaged items like meat and deli trays.
Systematically mislabeling prepackaged, WF-store-produced products to indicate greater net weight than is actually provided was, IIRC, the main method.
Having false scales for bulk goods would also be a possibility, though I don't think WF was ever accused of that.