I noticed that there is a difference in Signal Strength between the Broadcom chips (10, 15 series) and the Atheros chips (30, 45 series). Why does the 30AG report lower signal strength?

Answer

The calculations used for signal quality differ between the Atheros and Broadcom chipsets.  Broadcom chipsets report the actual RSSI value when calculating signal strength, whereas the Atheros chipsets report the actual RSSI minus relative noise. Therefore, the Atheros chipsets will appear to have a lower signal strength.