"Generally speaking" is pretty vague. If I correctly recall Thomas Sowell's discussion of this point, the shift out of the lowest-20% income group and the shift out of the highest-20% income group over a 10-year period is pretty significant, in the 25% range. These aren't all the same people trading places, of course.
The less ossified a society's economic opportunities are, the more likely it is that a person's income will reflect his most recent displays of wisdom or foolishness.
Such is the nature of a generality. The point is there will always be poor and rich (and those in between) and poverty can't be eliminated by redistributing wealth by force. Pointing out that the players in the play change roles doesn't alter the storyline of the play.