Right now I am trying Animals and Cars and almost all the words
are coming from the Animals one?

? Not sure why?

OH!!! I see what is happening..... Look:
Animals .... 52 words each about 14 times = 728
Car ... 25 words each about 16 times = 400
So, I think the computer should pick more words from the Car lesson
as there are only 400 points, where Animals has 728...
BUT.. the computer is picking them from Animals, because they only
have 14 while Car has 16 per word in the lesson.
***So, it is working correctly I would say***
I thought it was picking words by the total "score" of the lesson, but
it picks the words based on the score of the individual words in the
lesson.
P.S. One way to show this would be to add one more item on the
lesson page. Right now at the main lesson page the bottom of each
lesson says: "• Author: polyglot • Right: 1781.0 • Wrong: 9.0"
What I would do is add one more entry at the end called • Average: ??"
The average would be the number right divided by the number of
words in the lesson. Then you could see that even though a lesson has
a high score, it has a low average because there are many words.