An example of social responsibility

It happened when I was a high school student. One day, I heard a news that our local book store was doing a promotion that every books in the store was 10% off, and it dued tomorrow. So I planned to buy a exercise book tomorrow. When I was checking out, I asked the seller if the book take 10% off. And she said yes. I paid for it and left the store. A few steps away the store, i met a friend, she also just bought a book from that store and it was 20% off. I can't help myself getting angry. As a cosumer, I have the right to know the fact. Then I came back to the store and argued with the seller. Even though I won, she gave me 20% off, I still feel sad. How come she do it?