Officials said possible causes included the existence of empty areas inside the pyramid, internal air currents, or the use of different building materials.
There is an update to this story. A team using muon detectors have identified a large void potentially the size of the Grand Gallery and what appears to be a tunnel near the original main entrance. They are currently trying to design a microdrone capable of being inserted though a small hole with an inflatable balloon that will allow the drone to fly and take video of the space without significantly damaging the pyramid. I wrote a blog entry on this story with a link to a more recent news article: http://historyarch.com/2017/11/03/news-new-discovery-of-a-large-hidden-room-in-the-great-pyramid/
I cannot tell you whether Hawass is correct or not. As he says, scientists are not archaeologists and scientists may not understand how different sized stones might affect reported results. But archaeologists are not scientists which means the archaeologists may not understand how muon detection works and therefore they may be wrong. I think even the scientists are willing to concede that their approach and methods are new and unique and no one will really know until the space is physically explored, preferably in a non-invasive way.