It's a filter in the Forum/Thread List that allows you to be unaware all new content other than newly posted threads. It was a severe limitation in classic view that became, for some, a way to organize what they read in an orderly way. On classic tales, it didn't matter about old threads because they were gone permanently regardless of whether people were still wanting to talk about the topic or not. The filter option is called "First message". Since it hides content, the developers made it so it turns itself off automatically as is appropriate since you miss content if you leave it on mistakenly. Normally you'd use it for special purpose searches, but it just happens to match the order classic view had by limitation.
You're correct, it is indeed a parallel universe, but you would have to imagine you only used a really old piece of software that sorted things differently for many years. It would be confusing to see new content where no new content used to exist. You can kind of understand it if you think about your email inbox. Maybe a good analogy is to compare these off-topic threads to chapters in a book. You read the chapter and turn the page, you don't expect someone to add another paragraph to the chapter you read three days ago and to have that chapter somehow pop up when you pick up the book again, it is confusing because that was impossible in the old software. Of course, a dynamic discussion is not a cast-in-concrete book. Not justifying, just explaining as near as I can tell why people see new content bringing a thread upward in the list as a "mess" or "jumble". There is a sort of a consistency in seeing the thread list stay exactly the same as it was the day before, but that consistency defies staying current with the ongoing conversations.
I'm on the verge of making this option available to people in their preferences, so that is the only way they will see the forums. It won't have widespread use but those who want it, really need it to make sense of the forums.
The concept of a long running conversation did not exist in the old forums, but of course all forums nowadays have a few topics that remain interesting to people for weeks, months and sometimes even years. With this option set on, people will be left out of such conversations but it's their choice and they can also turn it off if they find it's leaving them out of discussions.
