Several reasons could answer your question, of course they are all hypothetical. One possible reason is that these reviews can be very bias, though I will say Yamaha is not typically one of the manufacturer's that they seem to levitate toward.
Another reason is that these measurement tests, as valuable as theu may be to some of us, are very taxing on an amplifier. Much more so than any real world listening scenarios. Your never going to see a 20hz-20khz sine wave sweep in a movie or music, much less at the volume they are trying to achieve.
Another reason could be that they are actually basing their review on sonic characteristics, rather than sheer power. It doesnt take much power before things start getting loud if your using average speakers, especially considering your not driving all 7ch with the same simultaneous signal.
What i thought was interesting was that the reviewer favored ypao over the audyssey equivelent. Which of course its just one persons opinion, but its not like you see these reviewers plugging ypao like theu do some of the other rc software