gst late fee

I filled one of my client's gstr 3b (nil) on 21st. The return was for nil there was zero liability of tax but in the "fee" column they charged 20 RS(10 in each cgst and sgst). Since amount was insignificant hence I paid it. but I was curious for what reason it was charged? It cannot be for late filling as it is minimum 200 per day for late filling of 3b and that too is charged in next month. Any idea?