Let's say your mother has blue eyes and your father does too, heck let's take it further, imagine you are from a rare breed who happen to not even have dormant genes for Brown eyes
But somehow you end up with brown eyes, a small mutation happened when you were just a shapeless bunch of cells during the DNA splitting, the wrong nucleotide was attached to the wrong gene and encoded the production of a totally different protein that the one you were meant to have
It's still "natural" because you were always that way
Sanji is the same here, he always had those genes, even before he was born he had them