Hi
its happening bcos of dynamic rate selection...
DRS is up-shifting and downshifting for rate optimization and improved
performance. The algorithms used for dynamic rate switching are
proprietary and are defined by radio card manufacturers. Most vendors
base DRS on receive signal strength indicator (RSSI) thresholds, packet
error rates, and retransmissions. Because vendors implement DRS
differently, you may have two different vendor client cards at the same
location, while one is communicating wit the access point at 11 Mbps and
the other is communicating at 2 Mbps.
there is nothing much you can do as it is something well thought by WNIC driver to run at 140 MBPS at that location ir-respective of location or how far it is from AP.