There are few points to consider that how Jetprivilege calculates the JP miles for each purchase.

1. Not all categories eligible for same earning of JP miles. For instance, on mobiles or electronics purchase you may get 2 JP miles per 100 spent. On fashion, clothing etc you may get 10 JP miles per 100 spent. It varies differently. Usually amazon publishes its referral reward structure per month to affiliates and jet privilege is simply passing us some of its benefit which it earns from amazon using referrals, hence the earnings may be different for same product purchased every month.
2. I have also noticed that while calculating the JP reward miles for each amazon purchase, they don't include taxes in overall puchase. If I purchase a product for Rs 10000 (including Rs 200 as tax), then they calculates the JP miles on Rs 9800.