generous "lowering of themselves" to be friendly or kind to the social inferior. Lady C does this, yet likes to have "the distinction of rank preserved" so I imagine she does this without ever letting you forget who is the superior.

In other words, he knows she is far, far above him and would expect to hardly be noticed by her (though it is usually to complete her table at quadrille!) so he sees it as such a kindness and generosity to be so acknowledged. IMHO, of course...