[Discuss-gnuradio] Sampling rate of gr-ieee 802.15.4

Hi, 

The 802.15.4 spec says that the channels should be 2 MHz wide with 5 MHz inter channel spacing. 

Why does gr-ieee 802.15.4 uses a sampling rate of 4MHz instead of 2 MHz ? 

Off course it works with commercial devices, but I am curious why 4 MHz and not 2 MHz. 

Regards

Sumit