That's only if you regard the viewpoint formats as prisons, which many do, which is a large part of the problem that you've run into. The formats are not a set of rules, as you know. They are tools. They are different ways to organize and present pov. So if the author makes the decision that he's not going to have any omniscience because he doesn't want to use pov in that way, but he wants to write in third person because he wants that effect over first person, he writes in third person limited. And if he wants to have the tools of omniscience in third person, a little or a lot, he writes in third person omniscient. There isn't any "slipping up" because it is not an exam. An author either chooses to use omniscient tools or not with regards to pov.