Are you referring to your average score on your account page? If so, I believe that's a standard average calculation, just a sum of your ratings / number of ratings.

Exactly. How come when I rate more TV shows my score just declines? It happens with the movies score too.

Well, depending on whether you're rating above or below your average, it's always going to change. Here are your 11 movie ratings. Your current average is 70% according to the website, and here's how it is calculated:

2.0 + 5.0 + 10.0 + 9.0 + 8.0 + 9.0 + 7.0 + 5.0 + 7.0 + 7.0 + 8.0 = 77
77 / 11 = 7
7 * 10 = 70%

Great, thank you.