I don’t understand right the calculation method. I selected a course with 311 lectures at LingQ. For the whole course the number of new words is reported as 64%. However, if I look at single lectures of that course, the rate of new words for each is usually 15 - 25%. So how comes that the whole course is reported with 64%???
if you have a course with 3 lessons each lesson has 10 words
1- lesson have 4/10 unknown words (40%)
2- have a 4/10 unknown words (40%)
3- have a 4/10 unknown words (40%)
And lets say that the known words in the 3 lessons were the same words but the unknown words where all different so you will have a total of 12 unknown words but just 6 known words so 12/18 unknown words for the whole course ( ≈67%)
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Hmm, OK. But any calculation of that kind makes this percentage quite useless for me then. I mean the goal of the whole issue is to help me in estimation of how difficult the lesson would be for my language level. So the calculation should be kind of an average percentage of unknown words I encounter on one page while reading - how often do I need to lookup in the dictionary. But like it is now, the percentage doesn’t tell me anything about the difficulty at the end.
It gives you an idea of how much unique words you don’t know in the lesson … and how much unique words you know …
I was confused about that, too, but figured it out with some help:
In essence, the answer is: you learn the most frequent words first - that means that chances are, many of your known words are in several or all lessons of a course. By contrast, it’s very likely that every lesson introduces many new unknown words. So your known words get counted in every lesson they appear in - but they only get counted once when you consider the whole course.
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Yeah, I understand. But as I told I don’t see much sense in these numbers then, because I still don’t have a clue of how hard the course would be for me 
More reasonable would be an average new words per page count or something like that.