I could never understand, & still don't, why the crevice volume is included in the CR calculation. By crevice volume, I mean the volume from the top ring to the piston crown. The theory is that the piston OD is smaller at the crown, but this expands from the heat under operating conditions & closes up this gap. Then you have the carbon formation on both the top of the bore & on the piston above the top ring; this would ensure no discernible gap.
