i have the same on my warrior

let me try to explain is easy:
take as example, with some paired swords your hits would be between 2000 - 4000 damage.
now what this ruby does is Ensure you more to deal stable hits which would be in THIS case: 3000.
so it just goes more somewhere inbetween your max & minimum hit so you are more certain of what you will hit. i tested this with epic paired axes in coba and hitted non stop 1662 dmg on goblins there with that ninja attack (6 attacks in rapid mode)