You asked: "The only thing I wonder about is why is there a functinal difference in humans? I wonder if that question has ever been answered, or if a theory has ever been proposed."
The absolute best [in my opinion] answer to those questions is found in Irwin Stone's Book, "THE HEALING FACTOR - Vitamin C Against Disease." You can get it free here: http://vitamincfoundation.org/stone/
You wrote: "I recently discovered that a very small amount of a natural source fully methylated B-vitamin in a nanized form (digested by bacteria) has been the most amazing B-vitamin in my life. I have able to...reverse grey hair..."
A few questions I have for you SB are:
1) What "kind of" reversal have you experienced, i.e. what percentage?
2) How much of each of the B vitamins do you ingest?
3) What age are you [IF you don't mind my asking]? (I am 56 years old, and roughly 30% to 40% grey).