Not surprising at all really. Parasites and disease is everywhere... The point of keeping up with water changes, and a good diet is to keep the fish healthy enough to fight off the parasites. It's like if you go to a mall and test 200 people, you will probably find millions of germs, and what not on them. By keeping healthy and eating right, your body fights off any attempts for them to bring you down. Same rules really...
So true . A healthy fish kept in a clean environment can fight off a lot. We under estimate how thought our fish really are.