why nvidia-smi show 1060 3GB max memory speed 4000MHZ?

As I know, nvidia 1060 used GDDR5 and memory speed reached 8000MT/s, for GDDR5, memory clock should be around 8000 / 4 = 2000MHZ.

For my 1060 3GB card, nvidia-smi showed Clock speed reached 4303MHZ, I am confused what kind of GDDR5 nvidia using. 

    Graphics                    : 1873 MHz
    SM                          : 1873 MHz
    Memory                      : 4303 MHz
    Video                       : 1683 MHz

 Below website also indicated that 1060 memory clock should be 2000MHZ.