I guess I wonder what they think a "thrifty" person/couple/family is buying.
I can't really do an exact comparison on our spending b/c my partner gets about half his meals at work. We average about $200/month, I suspect if he were to eat entirely at home we would be over $300.
We eat a lot of fresh produce, and although we don't eat much meat, we only buy organic/grass fed/most sustainable source possible. And we eat plenty of nuts, olive oil, chocolate, almond milk, random specialty ingredients, etc. I also include booze in our grocery spending, though we do separate out household supplies like soap, paper towels, TP, etc.
In the past I have fed myself on $35/week (about the level of a single female "thrifty" plan) and did not have access to either costco or the crazy cheap produce market we have here, just regular run of the mill grocery stores. I was eating a reasonably healthy diet, but it was not especially luxurious or varied. So I could see that these estimates are "thrifty" for people who may not have access to anything other than, say, safeway. Paradoxically, you kind of have to have some money/resources to be able to save money on food: you need to be able to get to or live near good options, and you often need to have a lump sum of money available to spend on buying higher price items in bulk. i.e., even though our spending averages $200, some months it is more like $350 because we stock up on a lot of items in a big trip to Costco. For us, that's no big deal, because we have the money and we know that it will balance out over time and save us money in the long run. But if we were living paycheck to paycheck there's no way we'd be able to afford that kind of stock up, so we'd be stuck buying food in smaller but more expensive quantities.