Hi . Let me answer this during the week, I left my Mac in the office.

, could you please specify where it is? I don't seem to find it and I can't remember of a "?" button

You can see it in the attached screen cap

It is supposed to show some explanation text on mouse-over event, it does for me in the 2.5.0.32 build.
The text is along the lines of: "Total files size is the size of all files in this backup. The real size of the data...."

Hey, cool, it works now. It must have been in one of the last updates.

Nice!