When a video card says the memory is 1300MHz (effective) what exactly does that mean?

I just bought a new video card that I thought had memory that was running at 1300Mhz but it only runs at 650Mhz according to Nvidia nTune. I was wondering exactly what the (effective) part meant after the memory speed.|||Your video card memory is DDR or double-data-rate memory. It achieves greater bandwidth than the preceding single-data-rate memory by transferring data on the rising and falling edges of the clock signal (double pumped). Effectively, it nearly doubles the transfer rate.|||EME 23150FC

The EME 23150FC is a ready to go 150 - 175 Watt 1250 - 1300MHz. Linear Amplifier. This amplifier requires 8 to 10 watts input for full output and features the EME-23150 silver plated cavity amplifier, built in relay switching (H402 2Z), EME Power meter with EME directional coupler, plus full metering including: Power output, Reflected Power, High Voltage, Plate Current, and low voltage DC supply.