^^You would. From the way these numbers are figured all we know is the total combustion chamber volume, that is head, head gasket and piston dish volume combined. If you'd CC the head you could eliminate it and would be left with the volume of the piston alone. Then and only then you could figure in actual compression from a piston change.
BTW, that Performance Hookup vendor has CC specs for all the pistons listed on their site.
Don't "think outside the box".
There is no box.