In the screen for switching between languages, a number is shown behind each which I assume is the number of know words? The count is wrong, however. For Japanese, it says I know 12 words but in fact I know 4. Even if you counted those on level 4/5, it would still only be 10.
Known words counter include words you have saved in Vocabulary that reached level 4 or checkmarked, plus words you marked as known directly in a lesson (blue to white), without saving them as LingQs first.
Ah, that makes a lot of sense, I must have done that twice. Thanks Zoran