IBT score: How the TOEFL Internet-based test work?

I was wondering if anyone here knows how the TOEFL Internet-based test work. How do they convert your essay raw score to the scale score. I remember the raw score for the two essays is 1~5 for each, but then the scale score for the section gives a score out of 30. I got 29, which is a good score, I believe. But I’m still curious, how do they get 29? If I got 5 for one essay 4 for another, shouldn’t it be 27?

TOEFL listening lectures: A lecture from a social studies class