If the PH and other conditions are good, I would say it's looking a little underfed. With low nitrogen you get lower leaf yellowing in the pattern you are experiencing, as the plant will draw from the lower leaves to feed the healthy top growth. If they were grown from seeds their genetics will each be a little different, so that one may just be a slightly heavier feeder. I would try upping the nitrogen just a little while keeping it lower than the phosphorus and potassium higher.