Coyote is right, sSolidWorks calculates only the volume of the parts, not the vacuum.
So we have to create another piece that represents this emptiness.
How did you do the Coyote boss? A boss with draft or a combination by subtraction?
Because I made a boss to the next surface and I get a slight difference with your result. It's not much (Volume = 1126.67 cubic centimeters) but it's surprising!