Behavior of how assignments units are computed has changed significantly between Microsoft Project 2010 and earlier versions. I've been seeing this question in forums recently and think that this post by Heather O'Cull:
http://blogs.msdn.com/b/project/archive/2010/04/29/assignment-units-in-project-2010.aspx
explains it better than anything else I've seen or could write. Long time users of Project should definitely read it to make sure they are aware of the new behavior.