Naga Siren's Illusion questions

I've noticed that as you level up Nagas ability Mirror Image, the Damage Reduction and Bonus Imcoming Damage both decrease for her illusions.

I'm mainly interested in what they do. Though the first one is pretty obvious on what it does I'm still curious on how they work overall, and how both work together in the one ability, Mirror Image.

As level increases 1-4
Damage Reduction 70%/65%/60%/55%
Bonus Incoming Damage 500%/400%/300%/200%

I've always thought that it just means her illusions take more damage (bonus incoming damage) and deal less damage (damage reduction).

the tooltip is kinda misleading since in every other ability damage goes up (30% damage dealt instead of 70% damage reductin).

Illusions deal 30/35/40/45% damage and take 500/400/300/200% damage.

I think it's logical that illusions deal 30/35/40/45% damage and receive 500/400/300/200% bonus damage, what's wrong with this?