The reason why Aeroflot doesn't put their planes under the RA- Register is because it is expensive to import Boeing products and then register them in the RA- products in Russia. So since Aeroflot leases their aircraft, they will put them under the registration that the aircraft owners originally put it under (like EI (Ireland) or VP (Bermuda. If you look at Aeroflot's Soviet/Russian builds, you'll notice that they are under the RA- register.