Known words in German

I am not very fussy over statistics and all as I spend more time just on reading and listening. However, I need to know why “my known words” counter was showing “3940” and all of a sudden it back tracked and now it is showing “3935”? Is it just a technical glitch or I simply do not understand the logic behind it.

One more thing, when I convert all my blue words into yellow ones and then when I am done with my lesson and click on “finish” button. I do not see any changes on my “known words” counter.

One time I forgot to convert a couple of blue words into yellow ones(meaning did not look up the words) and then click on “finish” button at the end of the lesson. Suddently, there was an increase in my “known words” counter by the same number. Logically speaking, it should be the other way around as I did not look them up and also did not know their correct meaning respectively.

I am posting this question for my own curiosity sake - it is not a big problem.