The exact same thing has happened to me, yet about 3 weeks ago. I have about 3.5GB worth of other and I know it is nothing on my phone. It is not jailbroken or anything. I just remember seeing it there and was like wtf?
I haven't tried adding loads of songs to see if it shrinks it, but it will be annoying if I have to do a restore.
EDIT - Heres' the thread I started when it happened
http://www.everythingicafe.com/forum...her-27437.html