I think the changes are due to two things. Last year when we switched to vBulletin I had added a little change to the portal so it would take you to the Lat Viewed Post in a discussion. When the software was upgraded earlier this year that "hack" was not re-applied and the links just took you to the first page of the thread. Then this poll
was posted and it was obvious that members liked the feature (which was unintentionally not added back). I added the feature back a few weeks ago. So that is why you have probably been seeing things differently recently.
The way you view discussions, newest on the first page, I can see how the default link would be good. Seeing as the majority or members use the standard view I think the Lat Post Viewed link is better.