Windows can consume space in a bunch of places that don't look like normal "files."
The first things I'd check are Windows Update cleanup, restore points, the pagefile, hibernation, and application caches. The WinSxS folder is another one people tend to blame, although you shouldn't manually delete anything from there.
I'd start with Settings > System > Storage and drill into the categories instead of immediately running a random cleanup utility. That should give you a rough idea of where the missing space is going.
