One of the possibilities for the difference in top speed may be from the fuel used during American testing.
America had a higher octane avgas than Japan. With the higher octane avgas, the Americans were able to get more power out of the engine (higher manifold pressure).
or
Perhaps it was just a typo that has been repeated over the years?
This article uses 705kph.
https://oldmachinepress.com/2013/09/...heavy-fighter/