Hello Cecil,
When we are calculating your cheapest shipping method, we take into account what the price is for all the items to ship together. For some arrangements of items, with certain weights and depending on whether they are "bound printed material" (books) there may be a shipping method that can ship all of the items together in one package as the cheapest shipping method. For other amounts and kinds of items in the same order, there may not be a shipping method that can ship them together cheaper than shipping them separately.
The system did calculate the cheapest shipping method it could offer, and this time it involved shipping the items in separate packages. The cheapest shipping method that could put them in one package was $2 more expensive than shipping them separately. I agree that some of the rules don't make logical sense, for instance until working in this industry I didn't know that shipping methods cared about how much of the package was books, and it is not always the sheer amount of items and the weight of packages that determines the cheapest way they can be shipped.
I'm not a postal wizard myself, so I'm sorry if my explanation isn't clear. If you have any questions or concerns, please let me know. Thank you!