Because part of the circle of the clamp is missing... If you were to snug-down the face plate without the bars in place, you could see and feel this without measuring. By the same token, when the bars are installed there is visible clearance between the faceplate and the rest of the stem.

While a non-removeable faceplate stem would be more forgiving of a size mismatch (it's designed to flex some), it's still not the best idea... The flexible side would accomidate the mismatch, the rear side will not flex (since it's held rigid by the rest of the stem) and thereby lead to point loading.