Why does my 16 GB iPhone 6 only have like 13 GB?

So if you add up the number of GB and MB you get around 12 or 13 GB. But my phone is a 16 GB phone so where are my other 4 or 3 GB?

Yeah I had the old iPhone five and went to upgrade it, cause it wasn't taking photos anymore. And then I got the new I 6+ and that wasn't taking photos either. But I guess what they had done was transferred all the information from the old phone to the new one so I had to take it an Apple store and they cleared all that data for me so I could take pictures. The way the technician explain it to me is that when you use all the 16 GB up or like 13 of it there's no way to reset the phone and delete that information, I don't know how they got all that information onto my new phone or why they would even transfer it in the first place