I didn't know myself the answer to that, even though I knew it happened so I posted a query in learn_support and got this. So, I made an attempt at 'writing' a FAQ (with the help of maltor's answer) that would be helpful. Here it is:
Why does it suddenly say ' previous day' instead of 'previous # entries'; is there something wrong with my journal?
There is no problem, that's simply how it works. The recent events (main) page is built from a cache of your 75 most recent journal entries and the earlier/later links are used to navigate through that cache. Once you reach the end of that cache the earlier link changes to a direct link to your calendar/day views.