If you look on the "Protocol" column you'd see that the clients use the 2.4GHz band and 802.11g.
The max data rate for 802.11g is 54Mb which you'd also see in the column avg Rate.
So either the AP is only set to 802.11g or the client only supports 802.11g OR the WLAN service is set to WPA v1 or mixed mode.
For 802.11n speeds you'd need to set the WLAN service to WPAv2 as v1 isn't supported with n rates.
-Ron