Dual strike and impale mechanics

Hey everyone!
I started making Dual Strike phys build in PoB and a bit confused right now.(PoE 1)

I got two weapons, main one-Sword and secondary-Claw.
When i add impales on passive tree or with supp. skill gem, "the Combined DPS inc. impales" is lower then "Hit DPS", because PoB minuses "Impale DPS" from "Hit DPS"

I know DS hit rolls separetely with each weapon, but after count as one combined hit.

So the question is Why in PoB "impale DPS" lowers my dps when i use Dual Strike???