I've noticed several times that my wireless bit rate changes regardless of signal level and quality... This has happened on more than one wireless card. Take for example these two iwconfig readings:

ath0 IEEE 802.11g ESSID:"bovcon" Nickname:"proust"
Mode:Managed Frequency:2.462 GHz Access Point: 00:0E:2E:3F:CB:EA
Bit Rate:54 Mb/s Tx-Power:50 dBm Sensitivity=0/3
Retryff RTS thrff Fragment thrff
Power Managementff
Link Quality=38/94 Signal level=-57 dBm Noise level=-95 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:183 Invalid misc:183 Missed beacon:2

and

ath0 IEEE 802.11g ESSID:"bovcon" Nickname:"proust"
Mode:Managed Frequency:2.462 GHz Access Point: 00:0E:2E:3F:CB:EA
Bit Rate:36 Mb/s Tx-Power:50 dBm Sensitivity=0/3
Retryff RTS thrff Fragment thrff
Power Managementff
Link Quality=48/94 Signal level=-47 dBm Noise level=-95 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:7 Invalid misc:7 Missed beacon:1

The first one has lower link quality and less signal and yet higher bit rate. Could someone explain this to me?