Which page is "most recently seen" is something only the browser program can decide. You might have arrived at this page from somewhere way around the Internet. If so, the Back button will take you way back wherever you came from, because your browser program remembers where that was.
Which page is "to the left of" the current page is something only the designer of the document can decide. On any given page of this documentation, the Left button will always point at the same page, no matter what series of other unrelated pages you may have viewed previously.
In this document, the convention is that 3.1 is to the left of 3.2, and there is no page to the left of 3.1, for example (see 1.2 How this document is structured for details). When there is no page to the left, the Left button is disabled: .
The and buttons are similarly related.