You are sensing,what we call in your community the Fine Print Charges.I have explained in great deal here What is "no-cost EMI" and its hidden charges? I am Noob in CC
But
There will be Principal, Processing Fee + 18%GST, Interest + 18%GST. See that interest component will be having additional 18% GST. Now the discount which you got is of almost no use and would have better of without EMI option as there was an discount of Rs X on one time payment. So the additional Rs X discount is just the EMI + Processing fee waived off you still have to pay GST.