Puzzled

So I go and reload my 'friends' LJ to see if there are new messages [because when I went back 25 messages, half the 'page' consisted of messages I'd read already on the prior page].

I see that intermixed with the messages that had been there are now another dozen messages. So what's up with that? If the LJ messages are listed chronologically, how do other messages get missed and now sneak in there, further down the page - and not end up listed as new messages up at the 'page' top?