Iperf thruput slower in N than in G mode?

Hi, I just set up my new N wireless network by using Netgear's wnr3500. However when checking the thruput with iperf, it's surprising that it's way slower when it's connected in N mode ( ~10Mbps) than in G mode ( ~20 Mbps). I was using this configuration:

IBM X60 ------(1 Gbps ethernet)------- WNR3500 -----------(802.11N, WPA2 or no encryption, pure N mode)----------Zotac ION-ITX (Atheros 928x chip)

All the drivers and firmwares are the newest available. It doesn't matter if I use different channels, WPA2 or no encryption, or pure N or mix mode, the max thruput I can get is always about 10 Mbps while I can get steady 20 Mbps in pure G mode. Also there are no other devices connected to the network and the Zotac is about 3 ft away from the router.

Does anyone have any ideas of what's going on here and what I should do to improve the thruput?

Thanks in advance.