Well, there might still be a non-empty recycle bin. And the folder System Volume Information which (for system disks) contains recovery snapshots which can get quite large. Aprt from that I don't really know. It can happen, though, that the amount of free space isn't accurate due to some file system corruption. In most occasions this is harmless but running
chkdsk should fix it.
In any case, if you want to install another operating system on it, I'd suggest you format the drive to start with a clean partition for the new OS. Then you also know precisely that nothing else is on there :-)
ETA: kquinn made a good point as well: The Master File Table (MFT) might get pretty large if you had lots of files on that disk. It takes up a little space + around 512 bytes per file. But for that to fill 9 GiB you'd have to have almost 19 million files. I know I regularly have a few hundred thousand but that much seems unlikely. Especially because all those files are probably not just 1 KiB each.