I love Duolingo fluency algorithm.
Yes, I've finished a tree having 1% fluency ;-)
Haha, yes, I finished my tree and got it all gold, and was still on 1%. I was so annoyed I flounced off to memrise for a few weeks. I then was forced to admit that Duolingo is a better tool, and when I came back was at 20% after a couple of days refreshing it. Now at 45% - it's a glitch, but a very annoying one!
Duolingo's "fluency" (yes, double quotes) is the most questionable factor in the site. First, because can be improved up to 80% without I even say one word. Second, because can drop to 30%, 20% (and so on); it is like someone had forgetting everything he learned (because he is not fluent anymore).