at 2.4GHz, you can move just inches or change the orientation of the receiving antenna, and patiently wait to get many averaged RSSI readings, and see large differences. In non-line-of-sight, this is often due to multipath (reflections) which can be constructive or destructive depending on the sum of the "rays" reflected. It's more evident in the frame error rates (not displayed) than simple signal strength. Reflected signals arrive at the receiver's antenna(s) at different times due to the different path lengths (from reflections).
This is why MIMO came to be - to take advantage of, or minimize the effects of different kinds of multipath impacts.