Points system. Just got Nature Unit correct but only achieved 4 points. Some others 10. Why?
Duolingo has implemented a new change as per which revision of already completed lessons will earn XPs as per the current word strength. The XPs can vary between 3 (for all strong words) to 10 (for all weak words).
I find that doing "Strengthen Skills" from the home page almost always yields me a few 10 point lessons. When they drop to 7 I go work on a new skill. It's a system that works for me.
I don't really strengthen individual skills unless they become un-gold or unless I personally feel like I really need/want to review it despite what Duo thinks. I've only had one skill become un-gold so far, because I review so much I guess.