Remember that the available balance is only an estimate. The final amount you receive is only calculated when you request a withdrawal, not before, and in my experience, you always receive more than the available balance indicates.
Also, since you haven't requested your first withdrawal yet, your interview hasn't been validated, so the system is currently showing your 30% tax withholding. Don't worry, when you request your first payment, Itch will validate your interview, correct the tax withholding, and pay you the correct amount.
In any case, when you get paid, you can download a document with the details for your review, but in my experience, Itch has never made a mistake when paying me.