in the Seventh book, Chapter "A Place to Hide" Hermione Tells Harry and Ron That She had Never Performed a Memory Charm Before, But She has Already Told Them That She Altered Her Parents Memories, How Is That?

When The Death Eaters Attack Them at the Cafe Shop, and Ron Says That He has Never Performed a Memory Charm before, But She Knows The Theory. But She Already Told them That She Altered her Parents Memory.... I Thought That She Didn't want to Say That She Performed a Memory Charm in front of Dolohov So That They Wouldn't Find out about her Parents... Help?