A little off topic here, are any of you guys noticing random interest charges on your cards?

I have been using PM for a few years now and I always pay the entire statement due.

Month before last, the due was around a lakh and I had made a payment of 85,000 before the due date and 15,000 four days after the due date, clearing off the entire due. They charged a hefty interest of 6692.41+ST for a four day delayed payment. Called them and after asking for an explanation of this charge, they reversed the charge.

Last month, paid a little more than the due amount, cleared on the due date but still I see an interest charge of 1887.66+ST. Just called their customer care. They didn't have an explanation but promised to send me a courier with the 'detailed workings' and would again process a reversal.