Thanks for using WinMiner and for the feedback.
WinMiner base currency is USD, that means that the balance you see in your account is guaranteed against fluctuations in price of digital coins. For this purpose we keep your balance in USD.
When you choose a method in which you wish to withdraw your funds we take the balance (in USD) and make the required actions needed to enable the withdrawal. In the case of a BTC withdrawal we need to exchange the USD amount into BTC this exchange entails a fee that in many cases does not come in the form of a "fee" but rather embedded in the exchange rate.

At the request of our users, we are now working on adding a feature that will allow users to choose the base currency of WinMiner, that way, you will be able, if you so wish, to keep your balance in BTC and withdraw in BTC without exchange fees.

Hope this clarifies