Why is the signal strength in the driver log larger than 0dBm for the CF Series?

The driver adds a weighting factor of 75 to the RSSI value for all APs that are on the preferred band and above the roam trigger. This weighting factor usually makes the RSSI positive and ensures that any APs seen on the preferred band will be used before anything on the other band (as long as they are above the roam trigger).