Product: Credit card

I have a Citibank XXXX cash card. I took out a balance transfer for the card. I also made purchases on the card. I pay off my purchases plus the minimum payment amount each month, by the due date listed on the statement. Yet, I still receive an interest charge on the next statement. Each statement that I receive explains how to avoid paying interest. It says, the following Your due date is at least 23 days after the close of each billing cycle. We will not charge you interest on purchases if you pay your monthly Citi XXXX XXXX XXXX XXXX plus your entire balance, excluding any Citi Flex Plan balances, by the due date each month. If you do not pay by the due date each month, you will pay interest on your purchases from the date theyre posted to your account. 
Each month, I pay the minimum payment stated on the statement plus the full amount of purchases by the due date. By the terms on the statement, I should not owe any interest. Yet, I am still charged interest each month. Ive contacted Citibank twice. On the second time, today ( XX/XX/year> ), the said that the purchases were being treated as a cash advance. But that doesnt make sense to me, and I dont see anywhere where it says that they can convert standard purchases to a cash advance, especially given the terms on the invoice. The statement says the following about interest for cash advances : We will begin charging interest on cash advances, balance transfers, and Citi XXXX XXXX on the transaction date. It does not say the same for standard purchases. I dont see how Im obligated to pay any interest on standard purchases IF I pay in full by the due date listed on the invoice, which I have done. The rep said something about how because I made a balance transfer, and am also making purchases that the purchases are converted to cash advances. But that is simply not stated anywhere, nor is that reflected in my statement. My purchases arent listed as cash advances on the statement. Theyre listed as standard purchases. The statement explains that interest is calculated using daily accrual method from the time the purchase posts, but it also says that none of that interest is charged if full payment is made by the due date listed on the statement each month. And each month, Ive complied with those terms. So I should not be charged interest on those purchases. 
Citibank advertises its double cash card as allowing balance transfers with no interest AND allowing standard purchases. Citibank also incentivizes standard purchases by offering {$200.00} worth of points if XXXX in purchases are made within the first 6 months. Nowhere does Citibank say that it will convert standard purchases to a cash advance for the purpose of charging interest. This explanation provided verbally by the rep is not written anywhere in the statement or disclosed before making purchases. Moreover, my other cards dont work like this. As far as I can tell, capital one does not charge interest on cards with balance transfers if payment in full is made on time. Nor do they secretly and without any indication, attempt to convert purchases listed on the statement as purchases to some other kind of withdrawal for the purpose of charging interest. 
I think theres something wrong and/or deceptive with what Citibank is doing.