Soldato
I see where he got it from, Nvidia.
Ouch. Looks like some games don't work well with it at all.
Sauce
http://www.geforce.com/hardware/technology/g-sync/faq
Just a guess but I'm assuming it's legacy stuff and certain games that limit FPS by default without any form of V-Sync. Might be just a case of it doesn't work well out of the box in those games. There are always exceptions