Josephus on the dimensions of the Temple compound

Josephus says that the royal stoa was one stade ("furlong" in Whiston's translation.) The Roman stade was about 600 feet. Yet the southern wall is 920 feet long. This has led many to conclude, reasonably enough, that the stoa did not extend the full length of the wall. 

Yet Josephus is consistently short when it comes to the size of the platform. In his Antiquities (XV, last chapter), he says its whole compass amounted to 4 stadia or "furlongs", one on each side. In his earlier Jewish War (V, 5.2), he gives the perimeter as 6 stadia, "including the tower of Antonia." In fact, it is 8 stadia: we can see and measure the walls. 

Josephus reckoned the width of the platform as one stade. When he says, therefore, that the royal stoa was a single stade long, he is picturing it as extending the full length of the wall. 

As to why the historian was so consistently mistaken on these dimensions, we cannot know. He was writing in Rome, presumably relying on the notes of the Engineering Corps. Perhaps on these points there weren't any. As for his memory, it may not have been much better than yours or mine. Few distinguish, in memory, between 600 feet and 900.