That is because the light skinned one is what she looks like without this mod. If you go to the images section you will notice that if you place your mouse over the pics they are marked as "before" while the dark skinned ones are marked as "after". If your default Senna does not look like the light skinned version it is because I am using XCE and Bellas Faces.
I just looked it up and it says that if you talk to her before she can leave the wedding ceremony and you tell her where to live, she will go there. I did this on my game, although I didn't know I had to.
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http://skyrim.nexusmods.com/mods/4708