That confused me on reading too, until I realized it is the drill size for the "CAP" portion of the blank. "METAL" just means a twist drill bit. The particular pen noted (I looked) has only one barrel piece so there will only be one set of dimensions and drill bits. Some pens have the same drill bit size for both the lower (NIB) and upper (CAP), but larger pens will often have different sizes of both the drill bit and bushing sizes.
Hope this helps.