the data AS use is based on some 200X traffic data, which at the time there’s not as much demand in china compare to now, if you look back 5 years, it’s really surprising how fast china’s flight demand grows IRL
Don’t forget that sometimes even more than 60% of the passengers are connecting. This may be especially a factor in Japan If you fly just PVG-NRT it fills 3 airplanes. If you add 5 connections on both sides it can fill 10.
Perhaps it depends strongly on the number of connections you have, I have 6 daily flights on NRT - PVG on Croydon, that are basically full both ways, about 80-90% of the pax go to/from external feeder. And another 10 dailys on HND-PVG, same percentage of transfer.
I remember somewhere in Wiki said the demand in airlinesim is country based, my understanding would be the PAX is only counted between China and Japan, and divide the total PAX to each airport in that country. For example, if China and Japan both have only two cities share same demand level, the 4 route will share same 1/4 amount of total PAX between China and Japan. So even two small airport in China and Japan may have some PAX.
In your case, then it will become odd, as both PVG and NRT is large airport, the PAX should be many. So maybe my understanding about this country limitation is wrong.