iPhone available space: significant difference between iTunes and iPhone

Solution 1:

I had the same problem too. My iPhone 6 showed free memory at less than 200mb and iTunes showed more than 5.6GB.

To correct this problem: go to your device (iPhone) and go to settings, iTunes & App Store, click on your Apple ID and select Sign Out. Go back and Sign in again. This is a lot simpler than restoring your backup.

Solution 2:

Are you using iTunes Match? If so, turn it off. Any locally cached files will be purged. iTunes doesn't see the Match downloaded music but only what it is configured to sync. I think this is why you see usage difference between the phone and iTunes.

Solution 3:

To clear out 'ghost' space from iOS devices, first backup your device to iCloud [Fully] then wipe the device & restore from your most recent backup.

iCloud does not backup any of the 'ghost' space & therefore does not restore it either.