Inconsistency between Task Manager Processes Tab and Performance Tab
Determining how memory is being used to what extent is not as clear cut as many users might think. I recommend reading this excellent msdn blog article:
http://blogs.msdn.com/b/tims/archive/2010/10/28/pdc10-mysteries-of-windows-memory-management-revealed-part-one.aspx
It was written by Mark Russinovich himself and explains everything in detail and much better than you could probably hope to have it answered here.