As others have said, I believe you just have alot of "cached" information. Emails, address book, etc. I also found out for myself at least, for example, if you started a video download of 1GB and only downloaded half of that, the full 1GB is still accounted for unless you delete the download. Like I also said, some installer packs probably haven't been deleted yet because Make more space always have apps listed that are deleted on mine. Also, internet explorer takes alot of space if you have alot of history.
Go through each app and setting(like automatic SkyDrive upload) and see if there are pending downloads, long history in IE, or that you've missed some things. If not, maybe the system haven't met the requirements to delete old files yet.