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:
Then, problem solved :-)