The RSSI for both cellular and XBee are dependent on a clean Fresnel zone.  The Fresnel zone is an invisible and roughly football-shaped region between two points transmitting RF signals to each other. 

A Good Fresnel Zone is shown as the oval in the diagram below:

When the Fresnel zone is obstructed it acts in much the same way as a partially clogged pipe.  As an extreme example, if you set the RF device on a concrete floor, the Fresnel zone is sliced in half and your ability to transmit data will be adversely affected.  An ideal Fresnel zone is unobstructed between the two points and is high enough off of the ground to prevent the Fresnel zone from bottoming out.  Other obstacles and RF sources can also obstruct the Fresnel zone.