Difference between 1oz copper thickness and 1.5oz finished copper for PCB fabrication?
1.5 oz. finished copper is what you get when you start with 1.0 oz. copper (35 &micro;m thick) and add 17.5 &micro;m of plating to it (typical for the vias and other plated thru holes).

If you want to end up with 1.0 oz. finished copper, you need to start with 0.5 oz. copper clad stock for the outer layers.