Supercell building with Avogadro-1.1.1

Problem in supercell building of Quartz {001} ?

I have downloaded cif file from the below link but now I am confused in the cif files because the atoms are overlapping when I am generating supercell. Supercell of 2 times in each direction by using Avogadro. Actual number of atoms are 72 in supercell but in Avogadro it is showing 80 atoms.

Thanks in advance.