I don't quite understand converting a poly to subdiv? I've been going through the spitfire tutorial but Kurts models seems to convert to sub div and be more rounded then mine?

As I understand it polygons to subdiv should subdivide a poly model by 2. Therefore a cube with 2 subdivisions along its width, height and depth should, when 'Convert/polygons to subdiv is applied, be converted to a cube with 4 subdivisions along its width, height and depth?

However when I do this I'm presented with a sort of rounded off cube but one with exactly the same amount of sub divisions (2) as I had originally?

If anyone could help me with this I'd really appreciate it.