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Then you're measuring wrong. Your headtube is (I'm guessing not at a right angle) to the ground.
You need to measure the distance between the straight line of your steerer/quill to an offset straight line that intersects your bar clamp. Which should equal 10cm (if my pygag is correct it'll be approx 5.67cm above or below where your bar clamp currently is).
EDIT: my pythag, not pygag.
tallsam
Horizontal/parallel to the ground, is what I meant.