Why do some people practice good oral hygiene and still have bad teeth?

It's not fair! I've also noticed that some people who rarely brush or floss still seem to have good health. For example, my SO doesn't floss and his teeth and gums look nice and healthy, while I brush and floss twice a day but still have irritated, receding gums. My mum also has this problem, even though she has good oral hygiene. Is oral health just hereditary?