The irony in some of these budgets is that you can eat pretty affordably...if you aren't overworked, and can afford to spend a huge chunk of your time cooking. But because you're working two jobs, this isn't the case.
Bulk rice and bulk dried beans are dirt cheap. Bulk vegetable oil, coconut milk, potatos/onions/garlic are all cheap. Homemade Thai curry is cheap and delicious, but ain't no one got time for that after working a double shift. Homemade Mexican rice and refried beans are likewise cheap and delicious, but they too take time.
And of course, if you're spending bare minimum on rent you're probably sharing a kitchen with a handful of other folks (or you only have a personal hotplate), which also sucks for cooking.
So if you live in a nice house with a nice kitchen and aren't overworked, yeah...food can be pretty cheap.