I don't know where they get those values, but they don't seem accurate at all.
When you buy something, you have two types of taxes: value-added taxes, which depend on the buyer's country and are added by Itch on top of the displayed price.
The exact amount depends on the buyer's country of residence and is usually between 10% and 20%.
For example, if you sell a game for $10, and someone from Spain buys that game, that buyer must pay those $10 plus Spain's VAT, which is 21% (there are reductions, but in practical terms they don't apply to Itch).
That customer must pay Itch $12.1. Of that value, Itch must pay $2.1 to the Spanish government, and of the remaining $10, administrative costs are paid, and the remainder goes to the developer.
When the transaction occurs in the US, it's different. In that case, the US government requires Itch to withhold 30% of the value earned by the developer.
Depending on the developer's country (and the treaties that country has with the US), this withholding can be a maximum of 30%, less, or even 0%.
Returning to the previous example, if you, as a developer, sell your game for $10 and someone in the US buys it, that person only has to pay $10, but you, as a developer, will only be able to receive whatever is left after the withholding. If you live in a country that doesn't have a treaty with the US, then of those $10, the US government will withhold $3, leaving you with $7. Of those $7, you must pay administrative costs, and whatever is left over is what you can withdraw.