What ever happened to natural instincts?

I recently got my cat back after 7 years (that all by itself is a long story).
However, this is not about the events leading up to the return of my little cat, but about what is currently going on in my apartment.

You see...
I already HAVE a cat.
Bentley is a large 20 pound 14 year old male (hey! he is NOT fat!)

Attessa (the recent re-acquisition) is a teeny 5 pound 8 year old female.
They knew each other many years ago when she was but a wee kitten.

Now, she's back and she is behaving very...like a cat I guess.
She alternates between hiding underneath the bed in the deepest darkest recesses as if I was going to flay her and eat her alive, to parading around my apartment hissing and growling (quite meanly, I might add) at Bentley.
Rather ungrateful I think.

But what I find really strange is Bentley's reaction to the whole thing.
Which would be no reaction.
None.
Zip.

She hisses and growls and acts all aggressive at him and he sits there calmly and looks at her as if she were mad.

He doesn't mind sharing his food, the bed OR his litter box.

Sometimes, while she's cowering behind the couch, he'll go looking for her. Once he's ascertained where she is though, he goes back to whatever he was doing (lounging or eating usually), and pretends she doesn't actually exist.

I thought that natural instinct dictated that he freak out and get territorial and spit and hiss and generally act pissed off about the little bitchy cat that has invaded his space.
Instead, he takes everything in with great calmness and resides regally over his space.

He's confident, I suppose, with the fact that if he WANTED, he could kick her ass and chuck her off the balcony.

The fact remains however, that he's not behaving like a normal cat.
Which, really, is why I love him so very much.