"Professor" Snape and Respect

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No: HPFGUIDX 98323

First, this conversation between Harry and Dumbledore from pgs 241-2 
of Sorcerer's Stone:

"Quirrell said Snape --"
"Professor Snape, Harry." "Yes, him -- Quirrell said he hates me 
because he hated my father. Is that true?"
"Well, they did rather detest each other. Not unlike yourself and Mr.
Malfoy. And then, your father did something Snape could never 
forgive."

Why would DD correct Harry and then go on to use just "Snape" in 
conversation? 

Also, even though we know Hagrid respects and admires Dumbledore 
more than anybody else in the world, he still calls him 
just "Dumbledore" in conversation. Just a note
. :D

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