Only thing I could come across after a quick search on web suggested that some US based credit card issuers use a two cycle system for calculating interest. If you pay your bill in full one month then no interest accrues on purchases however with a two cycle system you have to have two back to back months of balance being cleared in full to avoid interest charges. Where you dont have two back to back months clearing the balance in full, the credit card issuer then reaches back to charge interest for the "interest free" period.
Not sure if that is the explanation *shrug*