Difference b/w GSTR-1 and GSTR-3B

Sir I have received a notice from the department asking to explain me the reasons for the differences between gstr-1 and gstr-3b in FY 18-19. actually i have made b2b invoices in march 2019 gstr-1 fy 18-19 and amended the fy 18-19 b2c in fy 19-20 march 2020 gstr-1. will that be sufficient to explain sir. pls help me sir