Someone who eats 'healthily' is going to be much harder to geolocate than someone who eats 'carnivore' style.

I suppose you would look for the strongest source and use that. However I can't help but feel that they're measuring differences in identical isotopes and not relying on unique isotopes (IE of a different mineral) to do the locating. More potassium might mean the North American east coast and more calcium could mean the west is how their method reads to me. This means that they're calculating from the sum of the isotopes.