Calibration of Sentinel-1 IW image

I am trying to generate gamma nought using the calibration LUT’s for IW3 subswath using python. While, trying to find the corresponding LUT in calibration xml file, I am finding the LUT values are given for line -642 and then the next line is from line no. 19. I am unable to understand that why the line no. is given from -642. why the line value iz not given from 0 ?. Kindly help.