I wonder also if how the individual "approaches" the thread affects things. I know everyone is different in their viewing habits, but personally, I always start with a "New Posts" search, and then read each thread by clicking on the "First Unread Message" button (
) immediately to the left of the thread name (actually I right-click the button and open in a new window - I still haven't got into the tabbed viewing thing for some reason).
I know some people go to the last post in a thread and read backwards.