Just the Japanese being fussy. They probably have no idea that India has a lot if experience in radiographic or xray quality welding.Fabrication of the bridge using steel by welding or rivetting or nut and bolt is fairly simple tech and we, as in Indian companies have ample experience.
I cannot think of any area where extra accuracy is needed for its construction.
Main place where accuracy is required is ground levelling and laying of tracks. It needs to be arrow straight and not "give" under load of the train moving at high speed.
I think we are missing something here.
Probably Japanese have their own standards for certifying welders. Like in India we have 6G welding with vertical overhead considered as most accomplished and complicated.
Could be for tracks and bridges.