Here are two more possible explanations which are to be checked.
1. Japanese much better speak English than Chinese and some parts of their activity (professional) goes in English.
2. Japanese language uses several writing systems, one of them (kanji) is actually the same as Chinese one. Maybe it is recognized as Chinese by automatics? If it is so, all Japanese sites also counted as Chinese.
Well, you can easily meet people from xUSSR countries in internet speaking Russian, but mainly they speak Russian at native level.So I'm curious about the percentage of users on the Russian-language internet who are using Russian as a second language or lingua franca, even though Russian is not their native language.