Give the user the following information: 

To determine a student's percentile rank, the course calculates their Elo rank and compares that rank to all other users. The number of users, the difficulty level of the questions the user has answered, the number of questions the user has answered, how other students answered that same question, and the amount of content the user has not completed all factor into the percentile rank. There are also other factors beyond these. This assures that, for example, a student who answers exactly one question in the course isn't in the 100th percentile.

For example, if a student has completed four Check assessments (roughly 40 questions) out of the 200+ questions in Consultative Selling Skills, and is being compared to all students, including those who have already finished the course, it makes sense that they would be in a lower percentile than the results of their tests might indicate.