Why are total visit and visitor data not equivalent to the sum of each level 2's data?

While level 1 sites (L1) are completely separate from one another, that is not the case for level 2 sites (L2). When a visitor goes through several of them over the course of a single visit, that visit will be counted once for each of the L2s individually as well as once for the L1 globally.

Let's say a site with 3 level 2 sites gets three visits. The first visit goes through every L2, the second only goes through L2 "B", and the third goes through L2s "A" and "B".

In total, there are three visits for the L1 but 6 visits to the L2s if they are looked at individually. The latter is not an exploitable total and should not be used to measure your audience. As such, we do not recommend adding up visits from specific L2s, chapters or pages.

This is also the case for other non-summable metrics such as visitors.