Many people who are genuinely affluent, or even rich, can have business losses or an off year in their profession, so that their income in a given year may be very low, or even negative, without their being poor in any meaningful sense.
This may help explain such things as hundreds of thousands of people with incomes below $20,000 a year living in homes that cost $300,000 and up. Many low-income people also have swimming pools or other luxuries that they could not afford if their incomes were permanently at their current level.
There is no reason for people to give up such luxuries because of a bad year, when they have been making a lot more money in previous years and can expect to be making a lot more money in future years.
Most Americans in the top fifth, the bottom fifth, or any of the fifths in between, do not stay there for a whole decade, much less for life. And most certainly do not remain permanently in the top one percent or the top one-hundredth of one percent.
Most income statistics do not follow given individuals from year to year, the way Internal Revenue statistics do. But those other statistics can create the misleading illusion that they do by comparing income brackets from year to year, even though people are moving in and out of those brackets all the time.
That especially includes the top one percent, who have become the focus of so much angst and so much rhetoric.