Q: iphone storage - space taken up?

Hi,

Using my PC, I periodically cut and paste photos off my iphone to free up space for more photos. I keep photos and videos on there that I'm particularly fond of. I noticed the other day that the storage on my wife's phone differs to mine. Our phones are the same model (iphone 6) - the Used and Available amounts both total the same (11.6 GB) yet she has twice the amount of photos that I do. Why is my phone so full up!? It can't be apps that are taking up the extra space as the screenshot shows the apps in order of most space taken up - i'd have to have hundreds on there to account for the difference of 3.8 GB.

It's led me to wonder if it's possible that my phone is using up part of the hard drive by saved 'archived' or 'hidden' photos. Is this possible? If so, can i do anything to gain this space back?