FNB - 55 days interest free does not mean 55 days interest free?

I use my CC and then settle each month, and as far as possible within the 55 days to obviously avoid the interest charge. For this case, I will overlook the interest FNB charged me in February, and just accept that they were correct. But now in March they charged me again. A small amount but I ain't questioning the amount, it is the principle. I can show that early in February I settled my CC (positive balance), and then started using the facility again (putting me back into negative). But within 30 odd days later, FNB charges me interest.

Here is what they said when I questioned February's charge: "Should the credit card account be in a debit balance and the client chooses not to pay the full amount, on or before the payment due date, the client loses the up to 55 day interest free period and will pay interest on the full outstanding balance, from the date of all the transaction(s), to the date on which payment is received. Should the account not be settled in full, further interest will be charged on the remaining balance, from the date of the payment to the next statement date, as well as interest on any new purchase(s)/transaction(s). This means that again the interest-free period will be suspended and interest will continue to be charged from the date of each transaction on the account, until the full outstanding amount owed is settled." Did you know that 55 days interest free doesn't mean 55 days interest free?

Please don't bother calling me FNB, I already shared with you that I have accepted the defeat. I am sharing here, for the benefit of other consumers.