When analysing Chapters, why is the total number of visits different from the sum of the lines?

Totals in the table are called coherent totals. They provide you with accurate information not only arithmetically speaking but also logically. 

Consider, for example, that our analysis shows a table of 10 chapters. If we take the total number of visits at the bottom of the table, it represents the total number of visits that accessed at least one of the 10 elements of the table.

In so far as a visit can access several chapters during its course, a visit can be linked to several lines of the table. If these elements were to be summed as they are, the same visit would, therefore, be counted several times, making the calculation incorrect.

Therefore, when the total number of visits is displayed, it de-duplicates the visits and does not return a sum, but an aggregate. The Visits metric is therefore considered not summable in this context.