In the picture of post number 7, the deck surface appears to show markings from previous decking. Do you know for sure if yours was previously decked?
As for the numbers, I think I agree with KB on this one, or at least they are pretty close. If we treat that as a flat top piston, which typically has around 5 or 6 cc valve pockets, you would add the volume of that little step around the piston to the valve notch volume, and that is how they get 7.3 cc. But, the numbers say that "flat top" would sit about .018" above the deck, assuming 9.6" deck height, and the numbers measured from 72bluNblu confirm this on his engine. With those numbers in the calculator on the diamondracing.net website, it comes out to 9.99 to 1, and that is including .305" top ring land height.
Is this engine already assembled? If not, maybe you can test the half inch down fill volume and that will give you exact numbers. You would "travel" the piston down half and inch from true TDC, not just measure the piston to the top of the deck directly. Subtract measured half inch down volume from a calculated perfect half inch tall cylinder with your 4.08" in bore and that will give you the true cc of that piston. Then we know how close to 6cc or 7.3 cc KB was on their estimate.