"Toast and water, Chambers." The butler nodded, as majestically as Chris himself, to the second footman, who flew to replenish the silver mug, which had been Lady Catherine's when she was a little girl.

When Christopher had drained it (he is a very thirsty boy), he repeated the question: "Do you think you could be of any use ?" Mr.Chambers, the butler, never seems to hear anything that people say, except when they ask for something to eat or drink; and he does not often hear that, because he watches to see what you want, and gives it of himself, or sends it by the footman.

He looks just as if he was having his photograph taken, staring at a point on the wall and thinking of nothing; but when Christopher repeated his question I saw Chambers frown.

I believe he thinks Christopher presumes on Lady Catherine's kindness, and does not approve of it.
It is quite the other way with Aunt Catherine.

Just when you would think she must turn angry, and scold Chris for being rude, she only begins to laugh, and shakes like a jelly (she is very stout), and encourages him.