I think they should change the ranked requirement from level 20 to level 50. You might think you're learning the maps well enough, but there's no chance you've played them enough to actually know them well. Especially since a lot of the games you've played were probably not on the maps that are in the ranked map pool. You've got to, at the very minimum, know where the objectives are, the paths to and from each objective, what's on the floors above and below each objective...there's really a lot to it.

That being said, there's nothing wrong with losing in ranked and getting placed low. It'll give you an opportunity to learn the maps while you climb up.