That's because the 50 are no longer the 'new posts', but you can still see them and maybe more for the last day [if there's more than the 50 you saw previously]. The 'new posts' then become what you haven't already seen previously.
But a picture paints a thousand words ['#so why can't i paint yooouuuu#'] ... hmmm! Sorry, here:
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Now, bend over! [Only registered and activated users can see links. ]