But then why is there such a big difference between the specific gravity of the Moon (3.33 grammes per cubic centimetre) and that of the Earth (5.5 gr.)? Furthermore, according to the latest information (analysis of samples brought back by the U.S. Apollo astronauts) lunar rock is not of the same composition as the Earth's.

From your first link.

The answer is that the moon is about the same density as the material in the mantle of the earth.

It is the earth''s massive ferrous core that makes it more dense.

The Russian scientists and NASA (interestingly) are working on the same faulty premises for some odd reason.