Most likely it's in extra downloads from apps, cache, and temporary files. You can clear Cache from your Application Manager or in Recovery Mode.
Also remember that while you can move apps to SD card, the main file always resides in internal memory, and it's up to each app to 'decide' what content can be moved to the card (extra files, images, etc.), if any... and if the app updates, most of the time it moves back the content to the internal storage.