Actually, the anomaly is right above the Grand Gallery, and of approximately the same dimensions.
I have a theory as to what this could mean. Personally, I believe that the currently mapped network of internal chambers and passages inside the Great Pyramid was a decoy. Above this, according to my theory, is the real network of internal passages and chambers, an exact replica of what exists in the lower, false chambers. These chambers I suspect may contain the Pyramid Texts, and Khufu's true burial awaits discovery in the corresponding king's chamber of the upper network. His royal mummy and grave goods are likely there awaiting discovery.
The true passages and chambers would have been sealed off by Khufu's son and successor Djedefre, and there is likely no genuine entrance to them. It was simply sealed off within the masonry. Khufu and his architect were more cunning than we give them credit for. They have likely succeeded in deterring both ancient tomb robbers and modern archaeologists.