Billing

I recently app**** for a Standard Bank Istore credit card, upon taking out the card I agreed that I will pay back R1800 per month, Last month I received a statement indicating that an amount of R3661.35 will be deducted from my account, I called to query this, and the call center agent assured me that that amount wont be debited if the agreement was to pay back R1800, Today I see that they have in fact debited be the amount on my statement. I have agreed to pay back R1800 per month, On the 26th April they debited me R1800, Please explain why my account was debited incorrectly? and when i queried this i was told not worry about the amount on the statement.