The processing of a withdrawal necessitates a certain amount of asset conversion from fiat to crypto or from crypto to fiat, depending upon the asset you are withdrawing from the application management. That conversion has a cost associated with it which is applied to the funds being withdrawn.
In addition, the market price of Bitcoin changes between the time the withdrawal processing begins and the time you see the updated post-withdrawal balance. Note that the balances are updated daily.
Please refer to the Terms of Service for additional information.