Por que Hermione não voa em uma vassoura?

Hermione voou no primeiro livro, quando os três tentavam pegar a chave encantada e novamente sair:

"You drink that," said Harry. "No, listen, get back and get Ron. Grab brooms from the flying-key room, they'll get you out of the trapdoor and past Fluffy..."

Mas ela realmente não queria voar desde o começo:

"Stick out your right hand over your broom," called Madam Hooch at the front, "and say 'Up!'"
"UP!" everyone shouted.
Harry's broom jumped into his hand at once, but it was one of the few that did. Hermione Granger's had simply rolled over on the ground, and Neville's hadn't moved at all. Perhaps brooms, like horses, could tell when you're afraid, thought Harry...

E na DH, ch. 4; os Sete Potters:

'Miss Granger with Kingsley, again by Thestral -'
Hermione looked reassured as she answered Kingsley's smile; Harry knew that Hermione, too, lacked confidence on a broomstick.

Ela voa em uma vassoura quando ela joga Quadribol com Harry, Rony e Gina enquanto ela e Harry estão na Toca antes do sexto ano em Hogwarts. Mesmo aqui está claro que ela não gostou da idéia de vassouras:

Harry remained within the confines of the Burrow's garden over the next few weeks. He spent most of his days playing two-a-side Quidditch in the Weasley's orchard (he and Hermione against Ron and Ginny; Hermione was dreadful and Ginny good, so they were reasonably well matched) and his evenings eating triple helpings of everything Mrs. Weasley put in front of him.

(de Harry Potter e o Enigma do Príncipe, Capítulo 6: Desvio de Draco)

Existem alguns casos de Hermione voando em uma vassoura - alguns que vemos diretamente, e alguns apenas referenciados.

Em Pedra Filosofal, duas vezes no último capítulo:

They seized a broomstick each and kicked off into the air, soaring into the midst of the cloud of keys.

Algumas frases depois ...

Ron dived, Hermione rocketed upwards, the key dodged them both and Harry streaked after it ...

E finalmente, depois da sala de poções, Harry diz a ela para voltar, usando as vassouras:

... grab brooms from the flying-key room, they'll get you out of the trapdoor and past Fluffy - go straight to the owlery and send Hedwig to Dumbledore, we need him.

Na Toca antes da escola, no Enigma do Príncipe:

He spent most of his days playing two-a-side Quidditch in the Weasley's orchard (he and Hermione against Ron and Ginny; Hermione was dreadful and Ginny good, so they were reasonably well matched) and his evenings eating triple helpings of everything Mrs. Weasley put in front of him.

Para tratar da segunda questão, temos a seguinte passagem no Capítulo Nove de Pedra Filosofal :

Hermione Granger was almost as nervous about flying as Neville was. This was something you couldn't learn by heart out of a book — not that she hadn't tried.

Parece que ela não costuma voar porque, não sendo algo que possa ser aprendido em um livro, ela não conseguiu ficar muito boa nisso.

No filme Harry Potter e o Prisioneiro de Azkaban , quando voei em Buckbeak com Harry, tenho certeza que ela grita 'Mas eu não gosto de voar' como Buckbeak decai.