But above all, why in Transaction n°2 I paid quazi double 0.00038385 BTC instead of 0.00020 BTC? (I have a screenshot that shows 0.00020 BTC of Fee before clicking on send in Kraken)
You didn't pay 0.00038385 BTC. You paid 0.0002 BTC as withdrawal fee on Kraken.
The 0.00038385 BTC displayed on block explorers like Kraken is the fee paid for your transaction by Kraken. That's the fee paid to miners.
Take note that Kraken probably processed multiple withdrawals in a single transaction.