DDR Stands for Double Data Rate, Which means it effectively doubles your frequency by Transferring double the data per clock cycle.
CPU-z is going to give you the Actual frequency the ram is running at, While they advertise ram being 2133MHz! when it really is only 1066, But because of DDR it would be equal to having 2133Mhz of non DRR ram.
1066.5Mhz x 2 = 2133MHz
SPD stands for Serial Presence Device, which is in the RAM Module that tells the bios what setting to use, (I believe this is for compatibility). So you have to manually enter the desired frequency in the BIOS.
Your RAM's SPD is 2133MHz, So when you put it in you Motherboard, It sets the speed to 2133MHz just in case you Motherboard doesn't support 3000MHz, Or can't Recognize it on it own.
3000MHz is the Tested Speed with overclocking, Which is increasing Frequencies and Voltages and adjusting memory timings.
You motherboard supports overclocking so you can find some guides and get the MHz you paid for!
Thank you for the elucidation. That idea of doubling the frequency crossed my mind, but even doubling it is still much lower than the advertised 3000MHz. – Hong – 2018-12-14T16:24:21.940
1I edited my answer, my bad – Pancakedinner – 2018-12-14T16:30:45.430
Thanks a lot for the clarification. Your answer has explained everything. – Hong – 2018-12-14T17:10:49.217