I was at 50% fluency - now 45%...why?
I was at 50% fluency when I logged out last week. Now, when I log in again, I am only at 45%. Why?
As far as Duolingo is aware, you haven't studied or practiced French since last week. Therefore, you've forgotten some things and are not quite as "fluent" as you used to be.
Here's a post from staff explaining how the fluency score is calculated.
It's just one of many tools and incentives. Try not to get too hung up on it :)