My recent blog post on Aruba Networks Airheads Community. Never assume WiFi radios hear the same access point the same way. One device can be well within scope for WiFi VoIP while other devices can be outside of scope. Always best to confirm with device studies and understand what device is your lowest common denominator.
Good info! Wonder if it's the RSSI algorithm that is creating most of the disparity or is it really that much difference in uV's at the actual receivers?