github who am I? (OSX)

I found the problem, my Mac keychain had USER_B. I found it when I searched for "github" in the Keychain app. I've updated the question to reflect this is a Mac-specific problem/solution.

I had the same problem. I was able to find out what key has been used by git using following command:

ssh -v -T [email protected]

Then, I saw another key is being used by github; So, I added my default key to SSH Agent:

ssh-add ~/.ssh/id_rsa

Then, problem solved :-)