PVP HELP/UNDERSTANDING

Why do people equip Jeffery's dagger in there off hand as if they get that weapons buff and runes when thsy two hand the other??

Also how is it that some people can be dual welding and stagger me put of my attack when Im poised out of my mind two handing a grand sword but when I duel weld a stuff im the one who gets staggerd out of my attacks??