I think that's the question, and usually that's because they were born in the wrong place, at the wrong time or under the wrong regime. When their kids are born, it's a different perhaps better time, so better starting conditions, and they now have a chance to succeed. The anomaly is corrected.
But those are anomalies, it's exceptional. I think the issue is when you take two people born in the same town, at the same time, and both work equally hard, except one has the parents to give them a head start in everything and the other doesn't. Maybe one will eventually make an average salary in a local company, while the other will have his own company.