Product: Credit card or prepaid card

I paid off two of my Capital One credit cards in full -- - paying the entire " Current Balance '' amounts due on XX/XX/XXXX ; one account {$3900.00} and the other account {$2000.00}. 
Both accounts showed a {$0.00} balance on XX/XX/XXXX when I checked. 
This morning I went onto the Capital One app ( to check my XXXX XXXX XXXX XXXX ) and found out that I now have a balance of {$74.00} on one of the accounts due to an interest charge applied, XX/XX/XXXX. 
I called Capital One to question why, if I paid the entire " Current Balance '' due, not just the Statement Balance ( which is generally less than the current balance due ), was I charged this fee. 
I was advised by both the initial off-shore representative that this was due to the interest being calculated and applied after statement closing ( XX/XX/XXXX ). There was nothing further he was able to explain so I requested a transfer to his supervisor or an escalations representative to at least be able to request a review. 
I spoke with XXXX ( no last initial was able to be given per their policy ), ID # XXXX, who initially gave the same explanation as the first representative, but who placed me on hold while she checked to see if my account would qualify to have the interest charge waived, due to having paid the account in full. 
She came back on the line and advised my account did not qualify to have the interest fee waived. Since I have never been late with either of my accounts, have never went overlimit, have never asked for any fees or interest charges removed in the last several years on either account, I asked how this was determined. She advised it is a formula on the " backend '' and since it is an internal process, the company is not obligated to share this information with their customers. 
I requested multiple times to speak with her supervisor or somebody that would be able to explain their process in determining whether an interest charge can be waived, i.e. is it a certain TYPE of account that is eligible, does an account have to be open a certain amount of years ( mine have been open several years ), etc. 
She advised her supervisor/manager was in the administrative office and does not take any customer calls in any situation multiple times. I asked to speak with another escalations rep who could possibly explain the mysterious backend formula/policy (? ) that is checked when a customer asks for fees/interest charges to be waived, but was advised that while I COULD do that, since she was unable to transfer within her department, I would need to CALL BACK and request an escalations representative. 
I am hoping your agency is able to open dialogue between myself and somebody with Capital One to explain how they determine whether an interest charge is able to be waived ( this mysterious backend formula ) especially on accounts which have maintained good standing for several years, with THOUSANDS of dollars in interest paid to the company over these years ( at one point I believe I was paying upwards of {$100.00} + monthly in interest alone on these cards ). 
I was also advised by XXXX that the OTHER Capital One account that was paid off, which is still showing {$0.00} Current balance with {$0.00} due, will LIKELY have an interest charge applied at some point following that accounts XX/XX/XXXX closing date. 
I thank you tremendously for any help you can offer in having somebody with this company reach out to me to assist me in understanding why my accounts did not qualify to have this charge waived.