That's RNG, if you happen to get lots of SD procs during your pull burst then you would be nerfing your dps by using the best weapon in offhand.. On the other hand if you had few SD procs but many Bloodsurge procs you would see a dps gain by running best weapon in offhand. Averaging out these scenarios sees the offhand weapon as a small dps gain for the first 80%, though in a realistic situation with a realistic fight you'd be better off using the best in the MH.
On a basic patchwerk fight it very slightly favours offhand weapon more (as the sim shows), but in actual practice with the way most fights go you're pretty much never going to benefit from running best weapon in offhand. But it's really such a minor thing anyway, it's not like many people are running around with a large disparity in weapon quality.