Thanks for reporting this! It seems indeed when you use the Core Faces
option in the Automatic Dimensioning, then sometimes ARCHICAD snaps to the skin next to the core skin, therefore showing a higher dimension value than it's supposed to. We'll be investigating this issue further and try to come up with a solution soon. Until then, you can Dimension your walls manually, or use the 3rd option (Dimension Structures) when you want to automatically dimension the Core structure of your Composite Walls.