Rub-a-dub-dub

I have another question about men that I hope someone can give some insight to. It has to do with their bathtubs. My friend, who happened to be my roommate in college, and I were talking about this today and recalling that another of our roommates had a boyfriend who would leave the tub black. She always thought he was just disgusting (and truth be told, he was), but I think that it might have been attributable to this strange phenomenon that I have noted about many men. I have noticed that some men, no matter how well their normal personal hygiene is, turn their tubs black. And some men don’t. I have pondered over this for years. It has long been my hypothesis that the blackness is coming from the bottoms of their feet, but WHY I cannot fathom. My friend recalls one year when her 18-year-old nephew was staying at her parents’ house. He showered every day so he was not excessively dirty, but yet he would turn the tub black in less than a week. In his case, the blackness was also accompanied by grit or sand! She says it was as if he had gone off-roading without a vehicle. And the answer cannot be that it is because they don’t scrub their tubs often enough. My friend scrubbed hers at least once a week, but the guy would make it black again in a matter of days. Also, I personally have let two months go by without scrubbing my tub, but yet it never showed any black smudges. So what is the answer? Any thoughts?