Hi Harkiranjeet,
It is still larger but marginally, like 1.00005 so I guess this is just computational errors piling up and this can be ignored.
What helped me to solve the issue was 2 things. First I enlarged a bit the free space between the grating and input line. My dimensions are 7 microns device and 20 microns free space. Second, I increased the number of time steps there was insufficient number of them. I had 10000 and 60000 or 100000 solved the issue.
Hope this helps,
Natalia