The reason is because she didn't survey the school, but only surveyed her class, and to add to that It didn't say it was a random selection of students. Juliet concludes that short stories is the favorite type of book of the students in her school because 80% of the students in her class like short stories. Just because most of her class chose short stories she believes the rest of the school will have the same choice.
Because she only ask to her class and get 80%. If she wants to know the favorite type of book of the students in her school she needs to ask to the whole school
