If you inspect your Bitcoin transaction with a Bitcoin explorer such as mempool.space, you might notice something strange: The transaction inputs on the left side sum up to a higher amount than you intended to send while on the right side, there is a second address next to the one you wanted to send to. This is a very common phenomenon in the Bitcoin network, and there is no reason to worry. You did not actually send more bitcoin to someone else, but instead discovered an interesting aspect about the Bitcoin transaction model.
Contrary to intuition, there are no "account balances" in Bitcoin as with your regular bank account. Instead, Bitcoin has striking similarities to how cash payments work. To spend cash, you always have to spend the full amount of your coins and banknotes, since you can't cut them in half. For example, to pay someone 21 EUR, you could hand them a 50 Euro note and receive 29 EUR as change in return. The same happens in Bitcoin and your unspent transaction outputs (UTXO): When the amount you want to spend is lower than that of your inputs ("the banknotes"), the BitBox02 will automatically create a change output back to an address that belongs to you.
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You can find out more about this topic in our blog post on UTXO consolidations.