Points per sentence and session

how are points assigned to sentences? When I finish the sentence correctly on the first try, I have received 8, 12, 16 or 24 points. The points seem to change day by day. Yesterday and today I got only 8 points. The previous few days I got upto 24 points per sentence.
I have set 10 sentences per session. So two days ago I was getting 200+; today for a session I am getting 80+ points.
What is the explanation?
I signed up for 6 months, so not free. I think that allows me PRO privleges.

This one was answered in my response to the previous post.