Citibank — Interest on part payment

Address:

Thane, Maharashtra, 401107

I am holding credit card of various bank and have been regularly paying the tital amount due every month, recently due to dome commitment i could make fulk payment due as per ststement generated, i am shocked to know that i will be charged interest on total amount due that too at 2.5/ 3%.
Even members payment history is overlooked.
And also many banks give 1 point on every rs.100/- spend but while redeemin for gift voucher of rs.500/- one has to redeem more than 2000 points. Why? Banks must charge only equal points for same amount as we have spend rs.100/- for 1 point