PCGS slabs absolutely do fit in the NGC boxes. I personally use the NGC boxes because they are the best I've tried - they fit just about any slab, and are well designed. They are vastly superior to the blue Whitman boxes, which don't really fit any slab well.
You were possibly thinking of old NGC fatty slabs, which don't really fit as well as others. Or you could still be strung out on drugs. How ya feeling old man?
Better, but the blood pressure medicine makes me a little dizzy, but not so bad that I still couldn't kick your butt at pool you young whippersnapper you! Chris