It's a unknown mystery at the moment. The firmware update that Nokia is releasing will have more information and how to clear it. There's also an app on the marketplace that suppose to help clear up the other storage if it getting too large.
There was a tip from Joe Belfiore is to "Try turn-off skydrive auto-photo upload."
Other rumor is the if you updated your phone to Potico that the installer didn't get removed from the other storage.
We'll have to wait for the firmware update from Nokia to see we get the fix.