Your attacks don't deal a fixed amount of damage - they deal (for ease of numbers) 900-1100 damage..
A flat 2% increase would lead to 918-1122 damage.
While not hugely familiar with Warriors, I image that the ruby does the equivalent of making the damage range equal to 970-1070. It makes your average damage higher, but in a tighter range - you will neither hit as hard, or have the weaker hits either.
Thanks , so my follow up on this , is it actually bad to grab all the 3 rubies and have a constant value let's say 950?