Aiport Express 802.11n speed problem

I just set up my AXn last night and tried some settings. I found that if I set the radio to 802.11 (n only, 5GHx) mode. The connection speed show in the network utility is 270Mbps. But the Tx rate (Option + click on airport icon) is only 54mbps all the time. In the other way, if I set the radio to 802.11 (n/b/g compatible, 2.4GHz). The connection speed became 130Mbps. But I can get a much faster Tx rate now (sometimes reach 130Mbps). This also prove but actual download exercise

This is abnormal, right? Anything wrong with my settings?

Please help. Thanks.