Well, UNSCOM and the ISG gave us these reports. General Hussein Kemal sacrificed his life to tell us that and other secrets.
So the information was well known to all of those countries. Why we chose not to listen, I am not so sure. Where Bush, Powell, Rice, etc. received their suspected quantities from, I have no idea.
I've got to go with Occam's Razor on this one. UNSCOM quite clearly destroyed them. I don't see any reason for ISG to lie about the biological weapons either.
It makes far more sense than some conspiracy theory of a top secret massive project to move biological and chemical weapons to Syria. It also shows that there was plenty of evidence of the destruction of WMD in Iraq.