I reached 42% fluency in my Swedish course yesterday and I took a screen dump and sent it as evidence to a friend. Today I note I have gone back to 41%!! What's going on?
The fluency score will increase and decrease, depending on which skills and how much you've practiced. It will commonly fluctuate (once you've completed the tree) between 50% and 60% fluent, but not in all cases. It might be best to simply not pay much attention to the percentage, since it doesn't accurately measure your fluency.
Also, Duolingo's Help Center is a good source of answers for these sort of things: