I was just about to make a thread on something similar, and still might. My Photos app says I have ~106GB used.
I uploaded everything to iCloud, then downloaded it to my Windows PC. Total photos? 14GB.
So I see your 7GB difference and raise you to 92GB difference.
What the heck?!
Check this out as well. Apparently I've used 31.9GB but Photos alone takes up 106.66GB. I've no idea what to believe.
That's even weirder. At least the storage numbers add up on my iPhone. This isn't exclusive on this version of iOS either, since the days of iOS 10 I've noticed it on my iPad Pro.
long term bugs are the worst then, because they never get fixed
I've uploaded everything again to OneDrive, manually pulled files off my phone, used the Photos in Windows to import and have the broken iCloud as a last backup.
Now I've just deleted all the pictures off my phone and will use OneDrive as the photos client.
Apple make great hardware....but software? Hmmmm...