Safari Back Button goes to old cached page, os X El Capitan, iMac
I think the explanation is that Safari often uses the cached version of the page when you use the back button. If you want to force a refresh, you hit refresh. If you are on a forum and want to go back to the thread listing, you can usually click on a "breadcrumb" link near the page header. That will have the same effect as using the back button and hitting refresh, because Safari will understand your click to signal a new page visit.