iTunes says my iPhone has more free space than it does

iTunes seems to consistently show my iPhone 4S as having 1-2GB more free space than the Phone says. The Phone will say it has about 500MB of free space, while iTunes will show 2.6GB free. If I try and add any more than 500MB of content and sync, iTunes will then say there is not enough room to sync, meaning the Phone's number is correct. This happens almost every time I plug the Phone into iTunes to sync.