Think of nin and atk as base damage and crit and inj like multipliers.

Crit is straight forward, its your chance of getting crit or deal crit.

Inj diff between you and the other guy acts like a mutiplier. The higher the base the more damage you do. At the same time, the higher the inj you have the higher that base damage mutiplies. I think crit is always 2x non crit damage and inj adds another layer of mutiplier to the overall damage.