This is actually not unusual. Russians have traditionally supported the Serbs because they are Christians. Russians before Communism (and now after) are very dedicated Christians. And as far as I can tell from history have always supported the Christian side of any conflict too.
There’s no such thing as a “Christian” side, in the Russian worldview. At best, an Eastern-Orthodox Christianity vs. the other factions of Christianity.
The Russian Church has a history of curbing the spread of Protestantism, there.