My understanding is that pre-rendered web pages are not involved. What is "cached" is a function whose output is a particular webpage rendered on the fly so that "flag/unflag," current score and number of comments can be kept up to date.
In other words, the function fetches a particular group of articles along with the current information about them and generates the page. Some of those articles were known at the time the function was generated. Some of the articles are new or have undergone a significant change in status since the function was generated - I suspect these incur greater overhead.
At a certain point, the number of changes are are large enough that it is more efficient to generate a new function.