Its because BT4 comes with the mac80211 drivers pre installed & patched for better performance/injection. iwpriv works with iee.. drivers
If you want to play around with your power levels you will have to do the following
try some codes from here ISO 3166-1 alpha-2 - Wikipedia, the free encyclopediaCode:iw reg set <alpha code>
i use US and i get 27dbm, you are probably already on US at the moment, i am not sure which country code gives the highest output but if you do decide to test this please reply with your results.