1. The nominal data transfer speed of the 802.11g wireless networking protocol is 54Mbps.
2. Regardless of signal strength, the "Wireless Network Adapter" (PC) and "Wireless Access Point" (Router) will endeavour to negotiate a connection at the full 54Mbps. Only if unable to do so will a connection then be established at a lower speed.
3. To be honest, I've no idea what your "histogram" is, but if you go into the Network Connections folder and double click the Wireless Network Connection, it will show the connection speed and signal strength.
4. <<<Is there any logic to this?>>> Probably not, unless the signal is so strong that the adapter is being overloaded and the problem manifests itself as a weaker signal. Errors necessitating the resending of data packets could be weakening the apparent signal strength.