Hermione Jean Granger is not the normal, typical girl that everyone thought she was. Harry ,and Ron typically assumed that everything and everyone that Hermione has met, they know about. Maybe it's arrogance, maybe it's true-but for Hermione, she knew otherwise. Ron and Harry decided during 1st year that she must have been a muggle-born if she knew sooo much of the muggle world. For the sake of not arguing (and for the sake of not having to utter a word to that "despicable red haired bad-tempered of a boy")-she did not argue with them.
You see, the truth is that she is actually a pureblood by birth, but a muggle-born through her childhood. She was adopted by her distant aunt and uncle when her blood parents decided to give her up to muggles for her safety. You see, her parents used to be Death-Eaters but when they heard of Lord Voldemort's plan to take in all the Death-Eaters' kids to train them for his "personal army"-her parents decided that she needed to have a better future. Their wishes were to have her act like a muggle until she reached Hogwarts. Then become a proud Slytherin like they were. But Hermione had different plans.
It still hurts her to know that she, in a way, betrayed her family. She only chose Gryffindor because, deep in her heart, she knew that she will find someone there that will help her fight for her family. And that she was right of.