Some versions of firmware actually change the readings. Inside the card an analog-to-digital conversion takes place. The analog reading that the card gets and reports to the Aironet Client Utility (ACU) does not always use the same calibration that was used in some of the earlier versions of firmware. For more information on this, refer to the Cisco Aironet Driver and Firmware Frequently Asked Questions.
The signal quality varies on machine the same distance away
WLAN adapters (wireless card) / ACU (Aironet Client Utility)