This is a "bug" in the docs layout with the old theme. The reason it happens is that the left navigation bar is actually listing the headings on the current page, not the pages in the docs. The "SymPy Tutorials", "SymPy Guides", and so on on the main page are both links to other pages and headings, but the left bar is just giving anchors to the headings.
The new theme in the dev docs does this correctly, because the left navigation bar always links to other pages and the right navigation bar links to headings on the current page. The new theme will become the main theme as soon as we do the next release of SymPy.
Aaron Meurer