The sets mostly overlap. The top 10 of the richest people was the same in 2016 and 2017 [1]. These 10 people saw their net worth increase by $107 billion.
[1] https://en.wikipedia.org/wiki/The_World%27s_Billionaires#201...
I would say this is especially not true in the US where social mobility is notoriously bad.
"Several large studies of mobility in developed countries in recent years have found the US among the lowest in mobility." [1]
[1] https://en.wikipedia.org/wiki/Socioeconomic_mobility_in_the_...
One of the best studies on intergenerational mobility in the US, using all IRS records (not sure how hard they had to work to get those) shows that mobility in the US has been fairly constant for 50+ years.
[1] http://www.equality-of-opportunity.org/assets/documents/mobi...
To be exact, it seems to be saying that the richest 500 are x% richer at the end of the year, not that the 500 richest at the beginning of the year are %x richer at the end of it. These are two different things because they are two different sets of people.
In decades past the richest and the poorest people don't remain in that part of the income distribution very long; this is especially true in the US.