I don't know, I could make some guesses.
Maybe Russia isn't that Russy, they have always had less white ethnic groups and maybe they have admixture. Russia is only like 80% ethnically Russian, Belarus is 84.9% Belarusian and 7.5% Russian. If ethnic Belarusians are only modestly more intelligent than ethnic Russians, they still have 92.4% weighed down by 7.6% instead of 80% weighed down by 20%. Their other ethnic groups are also more european than those in Russia, like Poles (3.1%), and Ukrainians (1.7%), which I bet are smarter than Chechens and tartars.
en.wikipedia.org
en.wikipedia.org
