The estimates of scores are based on the performance of students who participated in a field study for these listening comprehension questions.
Those students took the test on computer. You took your practice test by listening to audio tracks and answering questions in a book. Although the two experiences are comparable, the differences make it impossible to give an exact prediction of your score.
The students who participated in the field study were volunteers and may have differed in average ability from the actual TOEFL test-taking population.