memory usage on ipod doesn't match what itunes says
I have a 5th gen 32 gb. When I look at the usage on the ipod it tells me that I've used 23.2 gb, with 3.9 free. When I connect to itunes and look at the data bar it tells me that I have 20.18 gb free. Everything has been updated and restarted and synced. Short of a factory reset, is there a way to fix this? the ipod won't let me put any of my longer, hd movies on it for offline watching.
iPod touch, iOS 9.0.2