You are both right, TMM and YR. You can run more compression on a aluminum head engine because the aluminum head flex's more, causing 3 cc's more of chamber. I put my go-pro hero 8 under my hood and when floored under load, you can see the head bulge up under the compression stroke above the cylinder of power stroke if you play the camera in super slow motion to the measurable of 3cc's. That is why aluminum heads allow more initial compression, though they give it right back throughout the RPM.
