Hi Corrie - They are two completely different user ids.
The user id in the job definition 'owner:' field is the account under which the job runs, on the Agent machine ('machine:' field).
The user id in the WebUI Credentials tab is used only in the WebUI, to manage "credentials for back-end servers" - in other words if you need to use a different account in the WebUI to the default one you logged in with. This is usually to make the Enterprise Command Line work as the commands you enter here are sent from the WebUI Enterprise Command Line to the back end AutoSys server to be executed.