Finally got an explaination from Advanta about the calculation method. I don't know if other CC Companies use a similar method.
IF YOU EVER DON'T PIF: They take the Average Daily Balance for the current statement cycle AND the Average Daily Balance for the previous statement cycle, total them and they divide this by the number of days in the current statement month. This is how my Average Daily Balance for the current cycle of $8,869 became the $12,856 that they used to calculate the interest charged. After this the Average Daily Balance would have been correct and I would have continued to pay the absurd 15.99% on the true Average Daily Balance.
I do have to give Advanta credit for agreeing to drop my interest rate to 10 point something percent starting today.
I feel seriously overcharged but will admit that I should have been aware of what formula Advanta was using to charge interest. I will check the other accounts in case I ever don't PIF an account that has purchases. (Instead of a low interest BT where the promo period ends).
I hope everyone else learns from my experience.
the barely legal 'double cycle billing' shennanigans. in any case, congrats on the lowered rate. these days there are plenty of cards out there that do use avg daily bal method w/o double cycle billing. carrying on a double cycle card should be off the table period, what a rip!
DCU EQ 5.0, Citi EQ 08 Bankcard, PenFed EX NG2 EX 08: AFCU, Amex, Chase, PSECU EX 98(?) TU 08: Barclays, Discover