Dead on. DC votlage to the radio has a very large impact on power out.
All the work done back last May I included that as many has the Wbrand
audio chip that would fry at 13V and they would know what not regulating
the voltage to the chip limits them too.
I do all testing at either 12.6 (charged gellcell) or 13.8 (automotive systems and
most bench power for high power radios. Astron and Samlex).
Since the power amp design is marginal over all DC power applied has far to
great an effect. So yes the numbers can vary widely and if the DC is not speced
as well as what band its going to be mostly guess. Also if you have five units
and the same power it all over the map.