PDF report, visits summary - "unique visitors" always 0?

Hello everyone,

I would like to understand the values given on the section “Visits summary” of the pdf report.
Since beginning of september I’m downloading a pdf report on a weekly base and today I noticed that the value of the “Unique visitors” is always 0, while “Visits” counts 13. And I know for sure that last week 13 different persons have been on the website as all 13 took part in a quiz and used a form to submit their data. This is how it looks like:

What I don’t understand is: what is the difference between “visits” and “unique visitors” - and how can it be that 13 visitors have not been unique?

This occurs not only in the last report, but in every single one - also in a report that ranges over all 4 weeks. Could anybody help me and explain it to me? :slight_smile:

Thanks in advance

see How is a 'unique visitor' counted in Piwik? - Analytics Platform - Matomo

also see How do I enable "Unique Visitors" metric for Yearly reports and for Custom Date Ranges? - Analytics Platform - Matomo