Your point is clear. It just doesn't make sense. If they're almost identical, then why would they sound different? Also, if they're almost identical, then why would the build quality be different?Just because their power OUTPUT is almost identical, doesn't mean they can't sound very different because of build quality. Besides, in case you didn't know, these are integrated amps we are talking about. So there is a preamp section that has nothing to do with their power amp section. This can and often will radically change how an amp, given the same power OUTPUT, will sound.
Finally, these are also two different models - one made in Japan and the other made in China. That's why the build quality would be different. So, my point makes perfect sense.