question about how "new posts" works

[MoeMentim (OP)]

this morning before work I logged in & saw a long list of new posts, didn't read any but thought yay, i have lots of reading material when i get off work, 12 hours later I hit new posts & there were 4.  i know i can go through the recent posts but wonder why the unreads disappeared before i read them.  I love this place & don't want to miss anything, just wondering how it works.  I know just logging in doesn't erase them because if i come back 10 minutes or an hour later all my unreads  unreads are still there. 

[bignasty]

it's a lot easier to just click "All unread" so the ones you don't read won't disappear. As to your question, I have no idea