I just posted some thoughts on how I think this is meant to work, here. In short though, it looks like if you are filtering by a category it only shows what has updated since your last visit, but without any filtering it shows everything. At least that’s what I’m getting, I can scroll through weeks and months of old posts in that mode.
Hmm, though now that I scroll to the very bottom, it may be capped by when it was first installed? Could be it only indexes back that far. I couldn’t find much info on it either!