The interest always start to accrue the moment you make a transaction. However the accrued interest will be waived if the balance is paid in full on the due date of the prior statement.

This type of language is on virtually ALL credit cards but hardly anyone ever reads it and most everyone ASSUMES that there is no interest when the statement is paid in full every cycle.

There is ACCRUED interest which is waived when statement is paid in full by due date. It is fundamentally different from there is no interest (which does not exist in CC world). However the outcome seems to be the same to most people not reading the fine print.

The Citi explanation is to explain that when someone has an unpaid balance on previous statement therefore the ACCRUED interest will not be waived. For someone who accidentally carries a balance on a card, it would be far better to STOP using that card until AFTER the billing cycle when all prior interests have been paid in full and an absolute 0 balance shown on the statement.

If you google "double billing cycles" you would learn more than you want to on this topic.