Nominal impedance is one of those things that different people have all different kinds of descriptions for.
Anything that is not 4-8-16 ohms confuses many users.
We have a loudspeaker that we origionally rated as 6 ohm nominal. I got all kinds of phone calls asking what size amp to use becuase they could not find a 6 ohm output spec.
Even though we gave a minumum and a showed an impedance graph, it remained a "mystery".
When we changed the rating to 4 ohms, those phone calls stopped. It was easier for people to understand.
even though nothing changed with teh actual loudspeaker.
The average impedance was closer to 8 ohms. So where do you rate it? We went for the next lowest number, just to be on the safe side.
And if you try to rate the sensitivity on power-what impedance would you use? the rating? Which one. Hence the reason for sensitivity based on voltage-let the user figure out what impedance they feel should be used.
Impedance is just another one of those things that "seems" simple, but really has a lot of different ways in which it could be read. A simple number with a complicated meaning.