Some of the butons on the menu are "New" and "Old"
"Old" forum view shows all threads sorted chronologically by the date of the thread starter.
I assume that is the view you are interested in in.
"New" forum view shows all threads sorted chronologically by the date of LAST REPLY even if the thread started 20 years ago.
This new view can bring stories randomly to the top, just because some of the stories received a reply within the last days.
I hope this helps to clear confusion!
Any time you find yourself in the confusing forum view just clikc on the button "Old".