If during a visit an user consults several pages, we will have a visit displayed on each page, although overall it is only one visit.
For better clarity, let's take an example:
A visitor visits the pages example::A and example::B which both belong to the 'example' chapter.
In this case, you will get the following table:
If you sum up the visits, you get two visits when in fact there is only one visit. This explains why this metric cannot be sum at this level.
If you look at the chapter analysis, you will get the following table:
In this case, we will display only one visit since it is the same visit that visited two pages of the same chapter.
Overall, the visit metric cannot be sum on elements that can be viewed more than once during a visit, such as pages for example.