Corsi, like many stats, is an observation... not the say all end all for facts. If a guy walks into a room 100 times and it catches on fire 100 times its reasonable to suspect that he is lighting it on fire, but you couldnt and shouldnt convict the guy without further proof.
When you look at possession stats, or plus minus, or even scoring...any stat really... it gives you a piece of the puzzle, but here is much you need to find out about a guy from watching him, verifying his linemates and system, and the opponants... if you go off pure stats you are just lying to yourself... now some stats will end up being an accurate reflection of a player, but again you have to watch to see...