Why does 128GB iPhone have 113GB capacity?
The other 6 GB are OS itself. The given capacity is a value for the physical storage that the phone has, not the amount that can be used by the owner. iOS is about 6 GB in size on its own.

Also, the formatted capacity is less than the size of the chips that are manufactured into the device. It's in a very small footnote at the bottom of the tech specs for each iOS device. Typically note #1



As you can see on the following link, each OS differs between versions, device type and device size so the gap in specs to usable space isn't a fixed constant.


How large is iOS?