In Harry Potter and the Philosopher's Stone, Ollivander and Harry have this conversation:
‘Ah yes,’ said the man. ‘Yes, yes. I thought I’d be seeing you soon. Harry Potter.’ It wasn’t a question. ‘You have your mother’s eyes. It seems only yesterday she was in here herself, buying her first wand. Ten and a quarter inches long, swishy, made of willow. Nice wand for charm work.’ Mr Ollivander moved closer to Harry. Harry wished he would blink. Those silvery eyes were a bit creepy. ‘Your father, on the other hand, favoured a mahogany wand. Eleven inches. Pliable. A little more power and excellent for transfiguration. Well, I say your father favoured it – it’s really the wand that chooses the wizard, of course.’
Later in the same chapter, Ollivander says this:
‘I’m sorry to say I sold the wand that did it,’ he said softly. ‘Thirteen and a half inches. Yew. Powerful wand, very powerful, and in the wrong hands … Well, if I’d known what that wand was going out into the world to do …’
This contradicts the claim that "wands are only as powerful as the wizards who use them. Some wizards just like to boast that theirs are bigger and better than other people's." Hermione says this. The Elder Wand is obviously exempted from this statement. Where does it say that all wands are equal in power, save for the Elder Wand? Why does Hermione claim that all wands are equal, save for the Elder Wand?