Pictures are worth a thousand words on this one.
The J key (older post) takes the user back in time one post at a time, looping from the bottom of one page to the top of the next older page:
The K key (newer post) however skips across the tops of pages in the reverse direction:
The K key should loop from the top of one page to the bottom of the next newer page e.g. 21 to 20, 11 to 10.
Looks like the url includes an anchor to scroll too, but #bottom isn't defined on the bottom post.