Default Missing Storage?

Hello everyone, I'm trying to find a solution to this but everywhere I look it always seems to be an issue from rooting. My phone is stock room, stock recovery and stock kernals. I haven't touched any of that at all with this phone. My phone says it has 9.23gb total storage space which obviously already takes into account the space the OS takes up. It says I have 2.24gb free but when I add that up with what's used I fall short of the 9.23gb by 3gb. Where has that 3gb gone? I am in need of that space. Also it says I have 435mb worth of photos and videos on the phone but when I add them up I only seem to have half that, the rest are on my sd card. What's happening there? Any help would be greatly appreciated. Thank you. Also when I only have about 200mb left on my phone and an trying to download some apps, sometimes ones that are, for example, 20mb will install (some won't) but at the same time some that are less than 1mb won't install and say insufficient storage space. How does that work? Thanks for the help.
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