Further reflection makes me think that it isn't so surprising that the number is so low, and this is not so much the good news that I thought. To qualify, you have to have 48 consecutive months of very low income. I can imagine a lot of people who are dirt-scraping poor who would catch a break now and then and exceed the cutoff for at least one month. Anyone who works in retail is going to get a boost every November and December when it's all hands on deck for the holiday shopping season. Same for delivery and warehouse workers. Agricultural work is highly seasonal. These sectors represent a very large segment of the workforce. Anyone who drifts from job to job could have some gravy months that allow them to catch up on rent and bills and IOUs, barely scraping by yet excluded from the Census Utterly Destitute list.
And, of course, income doesn't mean a whole lot by itself. You can have 48 great months in a row and still not keep up with interest on your debt. This is how many 100k folks feel, though on a case-by-case basis I would be tempted to laugh at them.