But synced data, I believe, only pertains to data, such as music, that has been saved to your computer and then transferred/synced via iTunes to your phone. My music is mostly years of ripping CDs via iTunes and then syncing that music with my phone.
When I unsynced that music/data, I didn't gain 133GB or storage (66GB for music and 67GB for synced content). I got 66GB of additional storage as the music was gone and the synced content category also went away.
So this leads me to believe that even though we see synced content listed in the storage bar and list under iPhone storage, it's not actually being counted against actual storage. It's misleading as heck and making many assume it is additional data taking up storage but now I am not so sure.