Looks like the credentials defined by doing People->User->Credentials are not available in jobs (for example when opening the drop-down menu in GIT configuration).
Step to reproduce:

• enable security and create an `admin` user
• login with `admin`
• browse to People/Admin and `add credentials` with no domain and global scope
• create a job and configure the GIT section (or the ssh-agent section if you had created an ssh key)
• open the drop-down menu for credentials
• all global and folder-specific credentials will be shown but personal credentials won't be there.

I would expect user-specific credentials to appear when the use configuring the job is the one the credentials belong to.
I have seen several questions on stack-overflows and google groups (e.g https://groups.google.com/forum/#!msg/jenkinsci-users/MkSJvvNFQCk/DAYdcIKaCAAJ) regarding this, but none of them has an answer.