Each person is different. Sometimes there is no explaining it.
That being said, bad (or good) health can be passed down by generations. So if your parents eat unhealthy they may not show the ill effects, but they passed down poorer health to you. Your grandparents probably gave a stronger foundation to your parents than they gave to you. If you continue to live in a state of poor health, then your children could be born in an even WORSE condition, unless you break the cycle.
I believe the Barefoot Herbalist (MH) has talked about this.