Citi is definitely not earning 5% via interchange fees, this is known. While they might lose some money on the 5% for some credit card users that use it correctly, it is most likely more than made up for by other credit card users who either use the credit card for other types of purchases and receive much less in rewards, resulting in a net gain.

Or citi has figured out that certain users of the card carry a balance and they spend more on interest than it costs in Citi in rewards. If the situation changes and Citi is losing money, then they change the reward amount.

It is almost certainly from compound interest.