Fried, I may be able to answer this.
For example, most Westen people buy meat importd from the West, which is more expensive than that from, let's say
Sudan or Egypt.
Western expats buy goods, like cereal, joghurt etc. imported from the West, not only those, but some. They are expensive.
Now ,non western expats may also buy western goods, but probably stick more to those items similar to or from their countries, they are much cheaper than those imported from the West.
Then we have different diets, due to taste, culture etc. For example, some Indian people are vegetarians (of course, people from the West are sometimes vegetarian too), so they buy veg imported from India, and those that are locally grown.
Now if you take a Western vegetarian,or non vegetarian, they might buy veg from Europe, and some locally grown stuff.
Thing is, many Westerners feel that the vegetables offered here ,locally and imported, are of bad quality, hence use a service,offered by some Expat ladies, that brings them fresh veg from Europe everyweek.

So, there are some reasons how food budget has to do with nationality.There are many more reasons, but it would take too long to post them all.

What comes around, goes around....