Genetics can be a funny thing.
My best friend is Puerto Rican, and his parents, brother, and sister are all light skinned (not in the very pale sense, but in the sense of being similar to say the middle of the Caucasian range, with his mom and sister being somewhat in the lighter end of that range.) His dad appears to be either mostly Caribbean Indian and some Spanish or perhaps entirely Caribbean Indian and as mentioned is light-skinned; his mom appears to be mostly or perhaps entirely Spanish.
However he is rather dark skinned. Moreso than anyone without black or East Indian ancestry that I’ve ever met.
Enough so that I have to wonder if his dad was wondering WHAT THE HECK??? in the delivery room 
Now before anyone suggests that well maybe his REAL dad is different from whom he thinks and from his brother and sister, but this is extremely unlikely as other than skin color and age, he is remarkably close in appearance to his brother. Much moreso than most brothers, closer than any two that I’ve known personally, though not as much so as identical twins. Split the difference between typical similar-looking brothers and identical twins, and maybe there you have it.
And there was a bit of a family repeat in this, though not to the same degree: As mentioned his sister would be in the lighter part of the Caucasian skin tone range. She married a Caucasian who is also pretty light. I would guess her husband to be probably mostly or entirely of English descent, or at any rate he looks it. Yet my friend’s nephew, from them, is considerably darker than either of his parents, though with the difference relative to his parents being only half as much so as with my friend. Still, it’s enough to catch your attention.
Or, historically, Carl Philip Emmanuel Bach was described as being quite dark, but his father (JS Bach) and mother were both light-skinned. So it’s not a unique thing.
Now, on your calves and the childhood walking style, you could well be right on this being causative. I am saying only that I don’t see how one would knowit (except perhaps if known from identical twin cases.) It reminded me of Steve Reeves attributing his calves to riding his bicycle up hills when he was a kid. How would he know how he calves would have turned out without having done that?