Why the difference between the RSSI and Rx Level measurements is so big?
The inverse square law attenuation of path loss or Friss Loss is the main factor followed by blocking effects of trees, or metal mesh or patio and then inverse phase reflection cancellation ( Ricean losses).  If 1W is +30dBm then it quickly drops in level to some distance ? 500m ? to -80dBm to noise threshold -90~-100dBm.   Depending on the RF band where speed and signal strength can affect performance tradeoffs, the % signal may be from 0 to 30% and excess signal strength from 30 to 100% , but this is just an estimate on my part.

Basically RSSi voltage is converted using log Amps with calibrated dBm and signal level in 0-100% is a linear conversion spanning more than 5 decades. (est.)