Bitcoin and cryptocurrencies are absolutely massive in Russia. I am surprised that Japan is the number one overall Bitcoin trader when there are so many Russians involved in the industry... you would think they would be higher up there.
This is complicated for a couple reasons. Firstly, Bitflyer (and likely other Japanese exchanges) offer zero-fee trading. That allows for a much higher level of churn (volume from the same traders). There could also be volume manipulation occurring, given that there are no fees.
Secondly, due to the unclear legal situation in Russia (with the government saying that the use of BTC was outright illegal in 2014, and banning access to many exchanges from within the country), most of their traders are on exchanges whose volume doesn't get attributed to Russia.