Many honestly think of soy sauce as a preserved foodstuff and that it will be fine when kept for a long period of time, but the truth is,
that after opening it, air gets to it, making it oxidized and thus darkening the color and diminishing the taste and fragrance.
Many people have experienced seeing wine becoming oxidized once opened, right?
Soy sauce is the same. If you compare soy sauce that has just been opened and some that has become oxidized, the difference in color is obvious and the taste is diminished.
Of course, ponzu is the same as well. Once air touches it, the oxidization process begins.