Why are Hermione and Ron so shady initially in HP6?

Currently rereading book 6 and this part(right when Harry arrives at the Burrow, pg. 90) caught me off guard. What were Hermione and Ron trying to keep from Harry in this exchange?

"It wasn't that exciting. [Dumbledore] just wanted me to help him persuade this old teacher to come out of retirement. His name's Horace Slughorn."

"Oh," said Ron, looking disappointed. "We thought--"

Hermione flashed a warning look at Ron, and Ron changed tack at top speed.

"--we thought it'd be something like that."

"You did?" said Harry, amused.

"Yeah...yeah, now Umbridge has left, obviously we need a new Defense Against the Dark Arts teacher, don't we? So, er, what's he like?"

"He looks a bit like a walrus, and he used to be Head of Slytherin," said Harry. "Something wrong, Hermione?"

She was watching him as though expecting strange symptoms to manifest themselves at any moment. She rearranged her features nashtily in an unconvincing smile.