See this may have many reasons. Some of the possible reasons are:

• More industrial appropriate students were in SRM AMRAVATI
• The number of companies visited the main campus got unsatisfactory responses from the students and thus to fill places the moderate students are taken up.
• Good students in Main campus may have not participated in the placement as a result the 100% placement ratio have reduced.
• The students in Amravati campus may get better exposure to the curriculum due to some good teachers or any other procedures.

These can be some of the probable cases. Actually, It can't be said what is the definite reason.

I hope my answer helps. All the very best for your future endeavors!