Don't overthink the FCR. You can get the same FCR from Purina or Optimal just by feeding different weights of feed. The difference between 1.51 and 1.42 when fed the same weight of feed is only about 300 J/gram (approximately 3% difference in digestible energy between one and the other).

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers