The tasks should be viewed as if they all belong to one big list. They are sorted by the day/time when they were added. The list is divided into pages to be more easily manageable.
So the pages themselves do not have any functional meaning, they simply aid the navigation through the big list, as they would in a paper notebook.
Use the right → and left ← arrows to go forward and backward through the pages.
To go to the first or the last page use ⇤ and ⇥ (home and end, respectively).
You can also use a shortcuts/menu options in Menu->Go to go directly to:
* the page of the last selected task (⌥⌘S) (for example if you want to continue selecting from there)
* the page of the last completed task (⌥⌘C)
* any page (⌥⌘G)