Discrepancy of free space on my iPhone and iTunes

Why does my iPhone show a different amount of free space on my iPhones Settings>General>Usage, than it does on iTunes after the iPhone is synced. The Usage on my phone continually shows about 1 GB less of free space than iTunes shows after it's synced. I have spoke with Applecare about this. Have been told there will be a little discrepancy, but not as much as I have. They have had me restore it with a previous backup. And it didn't solve the problem. Then they had me do a complete restore without backup. It solved the issue for about 3 days, then the discrepancy continued to grow until it was back to about 1 GB. With having only a 16 GB iPhone 1 GB represents a lot of space.

I am assuming the the amount of free space shown on my iPhone's Settings>General>Usage is more correct than what iTunes has.