Discrepancy in my iPhone Storage

Total Size = iOS + AI + System Data + Apps + Photos & Videos etc

Content categories 

The used content on your device is divided in these categories:

• Apps: installed apps and their content, and content stored in “On My iPhone/iPad/iPod touch” directory in the Files app, and Safari downloads.
• Photos: photos and videos stored in the Photos app.
• Media: music, videos, podcasts, ringtones, artwork and voice memos.
• Mail: emails and their attachments.
• Apple Books: books and PDFs in the Books app.
• Messages: messages and their attachments.
• iCloud Drive: iCloud Drive content that has been downloaded locally to your device.
• Other: non-removable mobile assets, such as Siri voices, fonts, dictionaries, non-removable logs and caches, Spotlight index, and system data, such as Keychain and CloudKit Database.
• System: space taken by the operating system. This can vary based on your device and model.

You can turn off a feature but unless it lets you actually delete the program files those will still take up space on your phone. I agree that that display is somewhat confusing. Since it refers to "iOS" it appears to be relating to the operating system files, but the Total Size is referring to everything. Perhaps a better reference would be using the method outlined in: How to check the storage on your iPhone and iPad - Apple Support