Okay, github uses some hash magic to figure out what account is affiliated with a particular ssh key. If you have two accounts using the same key, it has to guess (and does so poorly) about which user is trying to write.
Moreover, it tries keys somewhat sequentially. If the first in our ssh-agent (via ssh-add -L) is associated with a user, that's the only one it tries. If not, I believe it will go onto the next one (but again, if that second key is associated with multiple github users, it's going to hash to a specific user.) This is all because you can't tell github what user to use. De facto, the key is hashing to one and only one user and it ass/u/mes that is the user it will end up using.
But, the sad sad sad part is that it will not give you any more information.
You can set a specific "id" to use in our .ssh/config file like this:
Host github.com
Identityfile /home/USERNAME/.ssh/id_githubonly_rsa
