If you are wondering why your fluency drops even though you practice everyday? Well I just found out that I was 33% fluent in Spanish and now I am 28%. I found out that your lessons drop as you get farther and farther away from when you did them. When this happens you need to g back up to the lessons and strengthen them with the strengthen button at the top right of the page! Hope this helps!
I don't think that's the best answer from a didactic view, right Omega? The fluency meter is inaccurate, yes, and has arguably the wrong name, but in this case, worked as it should for Kayla: the number dropped, because she hadn't yet done any strengthening exercises, but now she has done some and sees it has a positive effect on the number, thus motivating her to keep strengthening instead of simply progressing through the tree as fast as possible.
So no, don't "ignore" it. Be aware of its limitations and what it actually represents - because it's hardly "fluency" - but used in the right way, it can be a valuable didactic and motivational tool.
You can also get quite lofty fluency numbers just progressing through the tree as fast as possible ;) I'd speculate it's the most time-effective way to go about it. From some other languages, the nomenclature is, usefully, not quite so illogical. Of course, these factors do not imply general recommendation of this tactic. I think most Duoling users practice far less than optimal for their goals.
What you said is in a sense correct.
Each word you've encountered has a strength value attached to it, from 0.0 to 1.0. In the Words tab if the strength is between 0.75 and 1.0, it'll show 4 strength bars (full), between 0.5 and 0.75 3 bars, etc. The strength value decays over time. When you re-encounter a word, its strength will be refreshed. If you get it right with no mistakes, its decay factor will be very small. From experience, it takes about 2 months to drop from full strength to 0.75 or a couple of days if you've made a mistake. It's likely the decay factor increases with time, so it won't require another 2 months to lose a second strength bar.
Your fluency score is then calculated using some formula that takes into account which words you know, how important they are, how well you know them and how likely you're to forget them again. This is why the fluency score increases so quickly when you first start a language, because you first learn words that appear in text all the time (verb to be, pronouns, etc).
If you like seeing numbers going up, you want to redo lessons that contain words with low strength. I've written a script that extracts that information from the API for my convenience. I've also noticed that some words appear only during general strengthening, so good luck reviewing those. And it seems each lesson has a short and long version. Some times the long version includes all of the words/conjugations/numbers/genders and some times not.
When people tell you it's a useless number, they mean that it doesn't actually reflect what a normal person perceives as fluency/mastery in a language. And no matter how well you know all the vocabulary that DL has to teach you (which is fairly limited in and of itself), if you move on from the service and don't use it for a while, your fluency score will naturally decrease. That doesn't mean you have forgotten those words, now, does it?