Heya there!
To figure things out, I would check your revenue settings found here: https://itch.io/user/settings/seller/revenue-sharing The default should be 10%!
Secondly, check your tax page here and see what the withholding rate is: https://itch.io/user/settings/seller/tax-information
Since that can often be the largest deduction of earnings depending on what it's set as, and if you've correctly configured that or not--and depending on your country, sometimes the withholding rate can be reduced or non-existent at all. Payment processor fees can also factor into things, too--though I'm not sure if it gives you a full breakdown of the calculations until you initiate your first payout.