When using scrollbar you will see the "No posts below this line. Probably you have read them all." when you reach the end of the feed.

But when using "n" key to navigate from article to article, you won't never know that you reached it. Nothing to inform/warn you.

It happens to me when I'm reading a feed where the last post does not fit in the screen (it may contain images or a lot of text, that prevent you to see the bottom of the article), you won't see the "end of feed" message.

Then you will try to press on "n" key again, because you are wondering if it's normal or not, or if the infinite scroll preoloader is currently processing new verbatim to display but take delays