I thought it was weird how even after being given a Deadly Upgrade and knowledge of instant kill spells, Eragon still had trouble with regular magicians, like the dwarf assassins or the guys at Feinster. Then I realized: Eragon might have more Manna at his disposal, and he might have secret Elven techniques, but those magicians have one thing he doesn't: years of practice.
Why Sapphira's egg appeared to Eragon. Bad/good luck. No, he was really the son of the one it was supposed to be sent to, and Arya was in a hurry. No, a hundred-ish Eldunari bent the magic to drop the egg off near Eragon!
I'm looking back at the series, and I deeply suspect that Christopher Paolini planned for the huge amount of WMG that Inheritance Cycle has generated. And he definitely left clues. He left so many for the Vault of Souls that people started assuming that the obvious answer was a feint. But Galbatorix's search for the True Name, on the other hand...