In the question, it states that Juliet wants to find the favourite type of book of the students at her school. She only surveyed her class and her class doesn't represent the entire school as it's just a small portion. 80% of her classmates might like short stories but the majority of the school population might not like short stories. For example, 56% might like young adult, 37% might like science fiction and 7% might like short stories.
So she can't assume that 80% of the school population likes short story when she only surveyed a small percentage of the population.
