It is entirely possible that those clients are not doing anything and are just sending management traffic back and forth.
It is also possible that those clients are just 802.11a or 802.11g clients, and that would be perfectly normal.
Lastly, the rate of frames is negotiated between the client and the access point. If frames are not successful either because of distance or retries, the access point and/or the client could attempt to send the frames at a lower rate, which is typically more reliable.