If you ever had Enable disk use checked on the nano's Summary tab (in iTunes), you may have copied non-iTunes files to it using Windows. You may have done this to transfer files between computers, like using a USB flash drive. If you never deleted those files, they will still be there, even if you later disable disk use. Those files count as Other. You can look by checking Enable disk use (if not currently checked), and looking in the nano (as a "disk") using Windows Explorer.
If your iPod nano is 5th gen, it as a video camera. I believe any videos taken (that were not deleted) count as Other.
If you can't figure out what it could be, you should Restore the nano. The Restore command is in iTunes, on the iPod's Summary tab. This will erase the nano, re-install the latest version of the nano's software, and set it to default settings. Anything that was previously part of Other should be gone.