Heh, it is the same reason why your fish have certain colors or patterns: Genetics.
The fishies that are bred are bred to have the white tips.. And as it is so common, its then hard to get rid of because there tends to be less fish -without- the white tips, so the genetics are then passed down from fishie to fishie, until it is so common place you go "wow" when you find a fish without it
Once again, Genetics. Some fish mutate so their color patterns don't exactly match.. One fish has it, the other does not, they are bred together, and sometimes you'll get oddities.