Summoner weapon question: Kelbim Bow vs. Final Stage Shadow Retrib

I know the conventional wisdom for summoners now is to use a 2 hand mage blunt over a bow. But why am I killing faster with a Kelbim Bow PVE than I am with the ingotted Shadow Retributer? Is the Kelbim the one exception to the bow vs. retrib rule? I'm testing it with robes (with retrib) and light armor (with bow) on a 91 Wynn Spectral Master on Genesis mobs. I am also switching out hair accessories (INT + m.atk and DEX + p.atk). Everything else is equal. My dyes are all +5 STR dyes. What am I missing?

Summoner weapon question: Kelbim Bow vs. Final Stage Shadow Retrib