As has been said above, if you have a double-sided, 3-fold or 4-fold document, you should add and remove pages by 2's, 3's or 4's respectively.
It might help to think about it like this: If you remove one page from a double-sided document you are effectively removing one side of the sheet of paper - or even just part of one side - and that's not something that you can do in reality.
You can add/remove single pages - and Scribus allows this because it's being flexible - but you shouldn't.
Always add/remove the number of pages that make up a "physical unit".