Originally Posted by Incredible Hans
I'd say that is one of the logical flaws of the movie.
Selvig's book tells us about Loki's character and what he did to Balder. Where does all that information come from? From the legends of the Vikings.
However, during the Viking age, Loki was still a child! In the movie Thor AND Loki weren't adults during that era. Was Balder killed by Loki when he was still a child? Probably not. Did Thor all the other things the Viking sagas tell us (fighting the giant Hrungnir, disguising as Freya, fishing for the Midgard Serpent) when Thor was a boy? Not very likely.
The Vikings had no real access to the personal lives of various Asgardians nor can we assume their historical documents are accurate.