Yup, that's very likely the thing.

In windows, "Domain" means "...the security database that hosts your user name and password."

In the case of machines joined to a corporate domain, the domain, user name and password are the corporate ones, i.e., MYCOMPANY\TFabris.

In the case of machines that have not ever joined to a corporate domain, the domain, user name and password are the local computer ones, i.e., COMPUTERNAME\TFabris.

The workgroup is not related to domains or security in any way, so the workgroup should not be included in the user name or domain fields at all. Workgroups only sort things into groups for browsing a group of computers.