Ravi, first of all, you should again re-calculate your marks using the image of answer sheet and the official answer key provided by the CBSE for the JEE Mains exam 2018. If you have given the offline exam conducted on 8th April then make sure you are matching the correct set answer key and if you have given online examination then again check the answers properly because even the same session answer key might not match with your answers because in the online exam even in the same session where questions were same but all of them were in different order for different student and even the options for each question were also jumbled. If you have given an offline exam then there could be one more problem that you might not have marked the options properly with the pen which might not have been read properly by the scanned machine.

If after re-checking thoroughly still, you have the marks difference then you can e-mail to the official CBSE email address or JEE Mains exam centre email address stating your query and sending the image answer sheet along with the result pdf. And request then to re-evaluate your marks along with the rank. I am not sure how effective this can be but still, this is the best you can give it a try.

Feel free to ask if you have any more queries. All the best!!