Backscatter value of SAR images

can anyone please tell me why does the backscatter of some features of SAR images comes to be lower than the theoretical NESZ mentioned with the sensor specification??

I have the same doubt. According to the documentation the NESZ is -22 for IW mode Sentinel-1 data.


When I checked the historgram of one the tile I found most of the data lie below -22 dB (NESZ) as can be seen in the image below.


Is it an issue with the calibration?