I have often received those when there was a change in the port taxes/fees. I think if they go up, DCL takes care of it, but if the charges go down, they refund the difference back to the customer.
It's pretty common to get a small refund as your cruise gets closer and port fees and taxes get calculated. They are just estimates if you book in advance.
It might be a refund of a port tax. I remember we received a nominal refund after our first cruise. My TA explained that the charge for DCL to visit a port (Nassau) decreased and it was refunded to us.
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