It goes by the last time you were logged in. So if you just logged in for the day, then it will show everything. If you go to the new posts then navigate away and then return a little later, there are now less new posts that have been posted since the last time you did that search. So even if you did not go to look at all the posts that were new the first time, it does a whole new search thinking you are done with the posts that showed up previously.