I think that you are confusing the _RATE_
or _SLOPE_ of each individually with
the differential increase per dB of input signal

"Ian Jackson" wrote in message
...
In message t, Old Ed
writes
The original author observed that many practical devices
(e.g., mixers) exhibit distortion levels that rise as the "power"
of the product in question. For example, third-order distortion
rises 3 times as fast (dB scale) as the desired (linear) signal.
Snip
Ed, where the increasing intermodulation distortion is simply a result
of increasing the level of the signals at the input of the mixer (or
amplifier), third order distortion actually rises TWICE as fast as the
desired signal. Third order distortion DOES rise on a 'three dB per dB'
basis, but the wanted signal also rises - at 1dB per dB. The difference
is 2dB. So the relationship is 2dB per dB.