iPhone missing storage

I got my iPhone 6+ in November. Is has been used regularly, apps have come and gone, but now there's something wrong with the storage. I recorded a quick video, deleted it, deleted it from recently deleted, and checked how much storage my device had left. 0 bytes out of my 16 were available. I proceeded to add the amount of space taken up by all of my apps, which is 8.056311GB. I then added the amount of space taken by the operating system to that, 4.58GB. I ended up with a total of 12.636311GB. And now, my question is why are 3.363689GB of my phone's storage missing?