Disappointed in Woolworths

I have a problem with my Woolworths store card. Can some one please explain to me, what is the point of paying an installment on the card every month.? I usually pay more than the installment they ask for. However, i see now for the pas two months that an amount of almost R400 gets deducted from my balance on my card, after payment was made? Why? Why then do i pay the installment? By doing this, i will never not get my positive balance up. As you will always take what is left in there. I need an explanation. Should these costs not be calculated in the installment??