I may be wrong here, and I'd welcome correction by the more experienced Zimbrians, but I believe that this graph only shows you what was marked as spam by SpamAssassin. The RBLs result in rejections that hit using a separate subroutine before SA gets a crack at the message, and thus they don't get included in the total.
Perhaps they ought to. . .it would definitely make our graphs (and those numbers we are asking for ) look more impressive, but I think that's how it works.
Damn! It would be nice to catch up the RBL rejections as well