according to
Samsung the GDDR5 memories utilize 20% less energy then GDDR3, maybe that's the reason to use GDDR5 memory.
and this
article tells us some more:
GDDR5 is designed in a way to only consume power when really needed. Several features and methods are implemented in a way to allow a demand driven power management:
* Extremely wide clock frequency range and data rates
* Multi level, demand driven termination enabling
* Low power modes for DRAM core
* Low supply voltage of 1.5V
* Data and address bit inversion
* Power-down and self refresh modes