I'm pretty sure the algorithm that's sorts people isn't the most fair one. You might have had the options if being put in Slytherin but there might be many people there already so it has put you on Ravenclaw. They might have also added some randomness to the algorithm, so people don't post stuff in the net like "What to choose to be sorted on X" (this randomness wouldn't be entirely random, it would keep in mind your choices and give more chances to the house you should be on).
This is all a theory, of course, since I haven't red the code I cannot be sure if this is correct. However, that is what I'd do if I was the developer.