Great question! The short answer is to make up for the losses in the transformers.
Many years ago, I was doing the calculations for distributed loudspeaker systems and then doing physical measurements to see the accuracy of the methods. It turned out the measurements were not very close to the calculations. The measure impedance at the amplifier was always less than the calculated amount. It turns out the part missing was the transformer specifications. Looking at loudspeaker specifications, many times there is no mention of the transformer. Every transformer has loss.
I'm a bit concerned about your comment about 1.2x multiplier as the math sheet that is given at the CTS-D test offers a 1.5x multiplier. I will have to review the Study Guide and see if we need to these into alignment.
I multiplier of 1.2 is really subtracting about 0.8dB and a multiplier of 1.5 is subtracting about 1.8dB from the output. Or, put the other way, the amplifier needs an additional 0.8 or 1.8dB gain to make up for the transformer loss. Based on my measurements, I would opt for the larger number. Remember, a 3dB change is doubling the wattage of the amplifier.
In practice, under sizing an amplifier can cause real problems. An under sized amplifier will clip and distort the audio signal. If clipping continues over an extended period, the loudspeakers can overheat and damage the coils. The amplifier will be drawing as much current as it can, overheating the amplifier circuits which can cause premature failure. The audio quality coming from the system will not sound very good.
I hope that helps answer your question.