Per https://collegereadiness.collegeboard.org/psat-nmsqt-psat-10/scores/understanding-scores the percentiles are
The Nationally Representative Sample percentile shows how your score compares to the scores of all U.S. students in a particular grade, including those who don’t typically take the test.
The User Percentile — Nation shows how your score compares to the scores of only some U.S. students in a particular grade, a group limited to students who typically take the test.
So it seems as neither percentile is an actual percentile based upon people who took the test. The National Percentile is how they think you stand up against all juniors. The User Percentile is how they think you might stand up to the typical junior who takes the test. However, it does not appear to be an actual percentage to be measured based upon this years results.