iPhone/iTunes Storage Discrepancy

Hey Community!

I've got a (hopefully) quick question. I've got an iPhone 6, 16 GB. It was a mistake going that low, but one I can live with. Anyways, because storage is at a premium I often run into the notifications about my storage being full. The thing is, every time I hook up my iPhone to iTunes I see a huge difference in available storage. For instance, right now:

As you can see, I seem to have 3.45 GB left when looking through iTunes. My iPhone tells a different story. It says I have 921.9 MB left.

I've sync'd my phone and backed it up as of right now. What is causing this discrepancy? What can I do to fix it? Is there anything to fix?

Thanks in advance!