If you recently updated or restored your phone, that's likely the system and app caches getting rewritten. Last week, as part of the iOS 8.2 update I restored my phone as new, and then reloaded the apps and data from an iTunes backup. That increased my free space from 5.5 GB to 7.1 GB, as the caches and a lot of downloaded app content got purged. Within a day, the free space had gone down to about 6.5 GB, as caches built back up and my iTunes Match directory got reloaded.
Also, if you use iTunes Match, any songs streamed from your playlists will stay on your phone for 30 days (unless you purge them by toggling iTunes Match on and off, or logging out of iCloud).
Also, over time the "Other" directory will accumulate garbage data that you cannot purge until you restore your phone. Much of this comes from botched downloads or interrupted syncing. If you do a lot of app and media downloads, or you don't have a reliable network connection, then the "Other" directory can grow quite large over time.