I learned that in sustainability class from Peter S (I also question its validity). I think the packaging and gas is usually much more. Although this may only apply to produce and it also depends on where the city and suburb is. For instance Madison will likely be more sustainable than L.A. because of the proximity to so many farms per capita.
I agree with your conclusions, especially that it's variable on the food item. I also think its variable on the city/suburb/etc. and its hard to draw any conclusion.