Why is my iCloud backup showing two different sizes?

Hi! So, I just backed up my iPhone because I plan to install new iOS on it and when I go to iCloud Backup section in settings, under “All device backups - (name of the phone) - This iPhone” it states that the backup size is 158GB and when I click on it for more information, it says the “Backup Size” is 170GB.

Why are there two different sizes! I just woke up, iPhone was backing up all night and it’s done with backing up. So, it’s not still in the process.