Well you heard close ...
The power output is determined by how hard the unit needs to operate to find bottom ...(Deeper water needs more power for the round trip of the ping duty cycle) ...
What you heard may be dealing with the factor that the HB units receive all echoes within the beam angle ...including the bottom, structure, fish, baitfish, clutter, water particulate, and denser water changes ...
The Sensitivity setting simply tells the unit how many of those total echoes you want to see on the display ... (On a Sensitivity scale from weaker echoes to stronger echoes) ...
Rickie