It's most likely caches and system junks. Your mms photos from the messaging app count as "others" as well
If you had an iOS 6 version under 6.1, there was a bug that even if you deleted the picture from the messaging app, the picture was still stored on your phone
The only way to remove these files legitly is to restore and start brand new. If you restore from backup, it'll restore these files
On the other hand, if you're jailbroken, you can access your phone's file system like on a windows and just delete those mms .jpg files and what not