iOS 9 iPhone 5c only 2,7gb of storage

Hey, when I thought about getting an iPhone I expected about 3gb of the 8gb storage to be occupied by iOS. Maybe another gigabyte for user data like Siri suggestions so I would still have 4gb left for my messaging apps, camera pictures, music, ... which would be enough for me.

However, I can hardly make use of 3gb; 2,789mb to be specific. The pic below shows what is being displayed on "Manage Storage"

I now even added up all the data in a spreadsheet and the result was the said total of 2,789. What's going on there? As I said, it would be reasonable to have like 3gb used up by system data. But selling a device that lets the user make use of 30% of the storage sounds awful to me.

This screenshot has been taken a couple of minutes after iTunes sync, so all the diagnostics logs (which I actually deactivated although that function doesn't seem to have any effect either) should be gone by now.

I seriously consider jailbreaking the device in order to get some insight of what exactly is occupying the space there.

It's already pretty weird that it shows a total of 5gb space on a 8gb device. But having control over only HALF of these 5gb is a disgrace. This is literally one third of the advertised storage space.

Any advice? I already made a back up, factory reset, and put the backup back on but that didn't help much as you see.