From my observation, "New Posts" doesn't show threads that haven't been updated since I viewed them, can go multiple pages, and beyond that show old posts. They also have just a bit more information in most columns. "Recent threads" looks to be one page of the most recent threads, whether I've viewed them or not.
I think both of them show any age thread, but only if recently updated.
I suspect you see that "LOL's Market Timing" in the "Recent threads" but not "New Posts" because you've viewed it since the latest change. If that's not it, please be more specific what you mean by "my feed" since I can only guess what you're referring to.