The reporting apps for storage are limited as to what partitions they see, typically they show only a few.
There are likely between 6 and 9 filesystems (aka partitions) on your phone to make it all happen - for the bootloader, the phone radio firmware, the protected areas (like /system that rooters so love to get to) and so forth.
That will add pretty much to 16, less formatting (which ought not be a large percentage).
You may notice similar "errors" in your ram tools, showing less than the actual silicon. Some is used by the Android ramdisk and memory mapping of hardware input/output, for example. (And the tools themselves run in ram, so don't expect any two tools to agree on the actual ram anyways.)
Anywho - the silicon is really there, the tools try to report what's useful to you.
Think of seeing how much disk space you really have after installing Windows. Just like buying a laptop, if you bought an "X_sized" GB hard disk, after Windows, drivers and the built-in so-forth, you're left with "X_sized - whatever" as your apparent free space.
Only difference here is they don't expect you to ask the real questions.
Bloatware is likely small, relatively speaking, unless you activate any of it and let it download the rest of itself. (usually the way)
