I wouldn't let the default percentages mislead you. Just under 1% of US developers or 7 respondents (EDIT: using the filters you defined) are making under $20k. That is likely well within any margin of error. We also don't know any of the details surrounding the responses. The numbers could be caused just as easily by working on an early stage startup, working part time, or even user input error as it could be actually making under $20k.