I would guess that in any given year a large chunk of the top 1% income earners were also top 1% income earners in other years. The median top 1% income earner, when calculated by the total number of living people who have ever had a top 1% income, may be a person who is only such an earner for 1-2 years. But it's fairly straightforward to see that this median can be reached while the majority of top 1% income earners in any given year are those who have been, or will be, top 1% earners in other years.

Basically, for a 30 year period, if a grand total of 5% of the populace will earn a top 1% income in their life, you could have 0.5% earning a top 1% income for the majority of their lives (so using up half of the top 1% slots). Another 0.9% earning a top 1% income for an average of 10 years of their lives (using up an additional 30% of the top 1% slots). And in the 30 year period this would leave only 1/5th of the top 1% slots open to the remaining 3.6%, who would have an average of 1.6 years each of being a top 1% income earner. Any spare slots would go to lottery winners.