Take for example the aforementioned grandparent: It's justice to redistribute wealth from those above some arbitrary line to those below (grandparent is presumably below it). That's only justice for him, it's bullshit to everyone else.

And you know what happens when differing senses of justice collide? Possibly made worse by fervor? Very Unpleasant Things(tm).

This is why we use legal justice rather than social justice: There is one and only one standard of justice that we all agree to accept after much deliberation.