If you are looking at your movie folder in Finder and adding up the contents found there as your 30GB of movies and then going to the storage information under "About the Mac" to see it indicating 80GB of movies this could be the issue.
Your mac will count every thing that has any file extension that qualifies as a "movie" and count it under movies whether it is in the movie folder or not. For example, mines says I have 4.5 GB of movies but in my movie folder I have two small files that together don't add up to half a gig.
Check your documents folder and your downloads folder for anything that resembles a "movie".
Edit: Nick types fast again! And he though of even more than what I did!
Do you edit videos in iMovie or any other video editing program? If you do the program will generate cache files that OS X will classify as movies. I use premiere pro and periodically I clear the cache. One time that cleared over 400 GB of hard drive space on a 1 TB drive.