You can say, "A has ten million dollars; B has even more - twenty million dollars." This is an emphasis, and even though 20 is indeed "much more" than 10, that is not an assertion made by the sentence. Yes, you might have been misinterpreting it.
Words and the way they are put together are not science. I would guess that 99% of spoken utterances are made without planning, and many, many things are written without real thought. Grammarians and lexicographers simply try to see some form of order in what native speakers produce.
I would suggest that you try to take a slightly more relaxed approach to the language. General rules are (generally) helpful. It's not the end of the world if not everything can be neatly slotted into a precise position in some artificial grid.