I've always found the operation of the recently added list somewhat strange, but for the past few days I've seen stuff appearing at the top of it that was not actually new, but that had been added a few days ago with more new stuff added after it.

Does anyone know what's going on with that?

Not sure what you mean. Can you screencap a example of it.

The recently added list on the side of the main page only shows recently added chapters that aren't recent releases, going to the seperate recently added page will show you the full list in proper order.