Why is the Total Unique User count different from the sum of unique user count in the table shown in Trend Insights?

Introduction
Cause
Conclusion

Introduction

When you apply the Unique Users metric to visualize your chart in Trend Insights, you notice that the Total Unique User count is not equal to the sum of the unique user count for each day in the Trend Insights table.

In the screenshot, the Total Unique Users count is 23, but the sum of unique users on each day comes up to 25.

Cause

Consider that an end user logs in to the application on Day 1 and also on Day 2.
Unique user count for Day 1 = 1
Unique user count for Day 2 = 1
But since the user is the same, the Total Unique User count is 1 and not 2.

Conclusion

If the same user logs in to the application on different days, they may be considered unique users on those specific days. But the Total Unique User count considers the user only once.