The reason is that the Unitas 6497/6498 is meant to be cheap and mass produced. Therefore the tolerances are big which is not good for chronometer precision. If you ask any watchmaker, he will tell you the same story. Then a 2.5 hertz movement is not as easy to regulate properly. The most important thing in this respect is the quality of the escapement and balance wheel/hairspring. Nivarox provides different quality grades for the Unitas and I think that the best grade have been used here.