Brom doesn't know everything the elves do and left out a lot of Eragon's education.
Except Oromis trained Brom, and there is a fairly major discrepancy there.
Oromis explains that there are actually no limits, but you can do magic by speaking which imposes limits on it.
Which means there should be no story. No limits means you can do anything. Eragon could just say Galbatorix's mountain and everything on it turns to dust, and it should be so.
We are explicitly told that you cannot tell a lie in the Ancient Language(or whatever they call it), yet Eragon proceeds to do so with his ballad.
And it seems that there's a difference between fictional literature and fact in the language, or there wouldn't be fiction for Eragon to read.
And we are given no evidence of this. The information we are given is: Lies=No. Truth=Yes.
No language is like that, though. A word is a word, whether it used for fact or fiction.
The elves have their own language. And they have the Ancient Language, unless I remember incorrectly.
In any case, there is a clearly defined mechanism for magic and it's history despite some minor inconsistancies which have less to do with magic and more to do with language.
Except that magic
is the language. Inconsistencies between the two shouldn't exist.