And efficiency refers to how efficiently the calories can be converted to body fat.
The easiest thing to convert to fat is… fat. No conversion necessary.
Second easiest? Sugar. Goes straight to blood glucose and thence to body fat.
Third easiest? Complex carbs. They turn to simple carbs and pick up the chain.
Hardest is protein. So that gets the best “efficiency” score.
And there you have it. SANE eating.