AR150 as AP unable to achieve 150Mbps

I am running firmware V2.2 on an AR150 and, via the HTML configuration, have set the mode to ‘11n (150M)’. This reports a wireless speed as 150 Mbps. After saving, the wireless config file (via SSH) reports:

config wifi-device 'radio0'
option type 'mac80211'
option channel '11'
option path 'platform/ar933x_wmac'
option hwmode '11ng'
option txpower '18'
option htmode 'HT40-'
option noscan '1'

However, the maximum bitrate available is 72Mbps. The advanced configuration also reports 72 Mbps. Any help would be appreciated. Thank you.