I think some questions are there for reasons other than to test if you know the best answer. They may be there for the following reasons.
Time Waster - To make you spend too much time on one of the questions that is not even graded. May not have any correct answers or may no make any sense at all
Distractor - Kind of like a time waster, but may be a grammar issue or a typo for us who just can't let it go.
Cheat Catcher - Part of an anti-cheating plan, I would explain more here, but... got to catch them cheaters.
Question Testing - Testing new questions. This one needs no explanation.
All Others - Because I have to complete the percentage pie chart with the ones I don't know about. I sure hope it is key lime, as I am hungry.