Yes, that is the correct Dagny Marie Valeur. The Digital Archives have her immigration date as August, not September. I have searched for William Liebrich, but there are several. I found one whose wife's name was Dagny, and she was Norwegian, though I never found them on Warnock Street.
I know "why" is extremely difficult to answer.
Thank you, though. (I knew you'd be one of the people to respond to my question!!)