While the reason is unknown, scientists think it might have something to do with the way DNA strands are made up within one's genetic coding. It's also believed that the multiple-moled are blessed with some sort of natural exfoliation system within the skin.
But the bad news is that people with a greater amount of moles are more likely to get skin cancer — and other types of cancer. So while you layer up on the sunscreen, make no bones about it: the study also found that people with a greater amount of moles have better bone density. Whee.