Basically a lot of game footage is being copyright flagged, in most cases its fine because the company will let you but will monitize it for themselves. Thats ok for Lets Plays because they don't really come under fair use because of the huge amounts of footage they use.
If it comes under fair use, you can dispute claims or if game company is ok with letting you record footage then you can dispute and the company will approve it.
gameinformer.com/b/news/archive/2013/12/11/tidal-wave-of-youtube-copyright-claims-underway.aspx
read that article.[DOUBLEPOST=1386989377,1386989250][/DOUBLEPOST]
The creators may have not registered those visuals with the content ID system yet or they don't mind. Some companies are happy to let people share footage without claims, some are not.