The 9 passenger thing is even more subtle than that. It's not how many passengers you have on board, it's how many you can carry in that configuration.
So, if you're in the 12 passenger configuration, but only have 9 passengers on board, you are still governed by the 12 passenger (i.e. no flight in the HV curve) limitation. If your configuration is for 9 passengers (and for the S-76, this requires a Flight Manual Supplement), then and only then can you use the HV in the performance section.
Conversely if you have a BK117 in the 'normal' (less than 9 passengers) then you can fly into the HV curve because it's in the performance section. Put the high density seating into the machine and the HV curve moves to the limitations section regardless of the number of passengers who actually get into the machine.
Subtle, eh?