There are costs for getting money in ATM machines. In the following receipt you could see -in red- the exchange rate from CUC to USD (EQUIVALENCIA), USD is the curency actually debited from your card. Additionally you could see the "service fee" -in blue- (MARGEN VENTA) applied to the transaction, usually 3%.
An ATM receipt.
If your bank charges an extra fee for extracting money from an external ATM (usually around 3%), you will be paying around 15% fees for extracting money in an ATM in Cuba.