how does the narrators behaviour towards a cat differ from that of montmorency ? what is unexpected in the attitude of the dog to the tom cat?

The narrator likes cats and Montmorency does not.

When I meet a cat, I say, “Poor Pussy!” and stop down and tickle the side of its head; and the cat sticks up its tail in a rigid, cast-iron manner, arches its back, and wipes its nose up against my trousers; and all is gentleness and peace. When Montmorency meets a cat, the whole street knows about it; and there is enough bad language wasted in ten seconds to last an ordinarily respectable man all his life, with care.

Montmorency runs head long into chasing the cat as if it was his only mission in life. The cat, in this chapter, stares Montmorency down and he backs away. Here is how Jerome imagines the conversation between Montmorency and the cat.

The Cat: “Can I do anything for you?”

Montmorency: “No—no, thanks.”

The Cat: “Don’t you mind speaking, if you really want anything, you know.”

Montmorency (backing down the High Street): “Oh, no—not at all—certainly—don’t you trouble. I—I am afraid I’ve made a mistake. I thought I knew you. Sorry I disturbed you.”

The Cat: “Not at all—quite a pleasure. Sure you don’t want anything, now?”

Montmorency (still backing): “Not at all, thanks—not at all—very kind of you. Good morning.”

The Cat: “Good-morning.”

Then the cat rose, and continued his trot; and Montmorency, fitting what he calls his tail carefully into its groove, came back to us, and took up an unimportant position in the rear.

To this day, if you say the word “Cats!” to Montmorency, he will visibly shrink and look up piteously at you, as if to say:

“Please don’t.”