Hi @Meigo, that extra data usage is generally made up all the other data usage sources that aren't large enough to be included in the main data usage categories. If you have donated any data, it will also appear in the other usage category.
For more info, I recommend checking out our Understanding & managing your data usage guide.
If you do have any other questions regarding your data usage, we'd be happy to help out. We would just need you to send us a message via the My Optus App so we can securely verify your account details.