under 'topic options' at the left top of the page next to 'new rply' there is a drop down menu which allows you to select the mode which the thread will be displayed. i used to notice that when i had 'threaded mode' selected, this would often happen for me as well. i always view topics in flat mode, so that way, you see each post in the order that is it is made, and when a response is made to a certain post, it tells you who it is in response to.
i don't know if that is issue, but you could check that out.
otherwise, sometimes people do delete posts, but even when they do, they cannot eliminate it because our ubb software assigns each one a number, and in that sense become part of a history. all that can be deleted is the text, but not the numerical position of the post itself.
sometimes posts are removed to an area for administrative discussion when there is something questionable. but again, even then, they cannot be deleted once they existed after the 'submit button' has been clicked.
i hope this helps,