The box I cobbled together from old pallets, dirt is a mix of potting soil and local dirt spiced with chicken poo.
For the more observant viewers, you may notice that the closest 'V' is slimmer than the upper square -- the pallets were a little short. I plan to plant smaller vegetables in those areas.
It looks like you measured from the outside of the box. The square feet areas are supposed to be just the dirt. If you cheat them in a little to even them out the planting will still work fine.