If you are on a Microsoft Windows client or a UNIX client, the operating system (OS) retains knowledge of the identity you claimed when you authenticated or logged in. That identity is published in your environment settings for easy access from scripts, but the environment can be changed by those scripts so that, judging from your environment settings alone, you can spoof a different user.
To see this spoofing, bring up a command prompt window (on Windows) and type
SET to see your settings. Perhaps you have a setting called
USERNAME near the end. Observe what it is. In the same window, type:
SET again and observe what the value of
USERNAME is. Note that this doesn't change your identity ...