Compression ratio is the ratio of the total cylinder and head volume w/ the piston at the bottom of it's stroke over the remaining volume of the cylinder and head w/ the piston at the top of it's stroke.
While I haven't had a Golf 10:1 motor apart, I do believe that the shape of the piston 'dome' is different, and subsequently decreases the amount of 'residual' volume when the piston is at the top of its stroke.
This volume represents a much smaller portion of the total volume than it does the residual volume, that's why the CR goes up.
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MARRS #25 ITB Rabbit GTI