Snape's Reaction to Harry assuming that he is a DE spy

[Miranda]

No: HPFGUIDX 108300

As a constant lurker, I haven't seen any recent discussions of this 
moment between Harry and Snape, so I decided to post and see what 
everyone else thinks.  I am curious about Snape's reaction to Harry 
assuming that he is a DE spy in OotP.  What does this mean in 
regards to Snape's work for the Order?

(page 521, Canadian Hardcover OotP):

"That is just as well, Potter," Snape said coldly, "because you are 
neither special nor important, and it is not up to you to find out 
what the Dark Lord is saying to his Death Eaters."
"No - that's your job, isn't it?" Harry shot at him.
He had not meant to say it; it had burst out of him in temper. For a 
long moment they stared at each other, Harry convinced he had gone 
too far. But there was a curious, almost satisfied expression on 
Snape's face when he answered.
"Yes, Potter," he said, his eyes glinting. "That is my job. Now, if 
you are ready, we will start again.

Why would Snape be satisfied if Harry guessed right and he is a spy? 
I have always assumed that this reaction means Harry is wrong, but 
what else could Snape be doing for the Order? Any ideas? Or is this 
satisfaction because Harry has figured things out? I've heard some 
theories that perhaps Snape is satisfied to gloat over Harry how 
much he risks for the Order, and that he knows more than Harry 
does.  I like the idea, but is it too obvious?  Any suggestions or 
directions to an earlier discussion on this topic would be 
appreciated.

Miranda