In order to process a deposit, a dynamic portfolio has to be created that is composed of both Bitcoin and US Dollars. This necessitates a certain amount of asset conversion from fiat to crypto or from crypto to fiat, depending upon the asset you are depositing. That conversion has a cost associated with it which is applied to the funds being deposited.
In addition, the market price of Bitcoin changes between the time the processing begins and the time you see the updated post-deposit balance. Note that the balances are updated daily.
Please refer to the Terms of Service for additional information.