Was using the “looking for upgrade” feature… for Naxx items… and MrRobot said that a one hand dagger (Midnight Haze) was about a 0.78% DPS improve for my char… over the Nefarian Staff of the shadow flame… so I assumed it meant I had an off hand in my bag… to pair with the dagger to give me the 0.78% improve.
So I got the dagger… but now when I use the “best in bags” option… it does not say the dagger is an upgrade… and continues to recommend the Staff of the shadow flame as best.
So whats gone wrong here? I presume I have misinterpreted/misunderstood something… but I need know … so I dont make the same mistake again.
I presume it simple… but I can add more details if needed.
Thanks in advance for enlightening me 
Upgrade finder, when looking at a 1-handed weapon, will pair it with a similar quality off-hand to give it a “fair” ranking. Otherwise if you only have a mediocre or bad off-hand in your bags, it would never recommend any 1-handed weapon as an upgrade, even if doing so (and also getting an off-hand) would be a good upgrade.
Caster weapon combo options have always been one of the most difficult things to rank – this is the best approach that we have found for it over the years.
Short version: get a good off-hand and then best in bags will choose your new main hand to go with it.
Thanks for the reply… I had wondered how you handled the one hand/two hand scenario you describe… and agree, its a tricky one.
But thanks for the explanation - now I know