I didn't know that, and you're right. For anyone who's interested, I found the following references regarding the $8.39 additional fee for a .com registration:

https://itp.cdn.icann.org/en/files/registry-agreements/com/c...

https://www.icann.org/en/system/files/correspondence/stewart...

https://www.icann.org/en/announcements/details/icann-and-ver...