Microsoft blames common characteristics in Silverlight 4 and it's predecessor for the documentation overlaps. This begs the obvious question, why release a new version if it's som similar to the old one.
"There is quite a bit of overlap between developing for Silverlight 3 and Silverlight 4 and so in order to not duplicate content in different locations on the web, both versions are covered in the same set of MSDN documentation," revealed a member of the Silverlight team.
Well don't expect this to be resolved anytime soon. Perhaps next time the Softies will get the documentation squared away first.
Microsoft leaves something to be desired in the online documentation of Silverlight 4.