The special status of the built-in ADMIN user is due to how the Laserfiche server interprets the login for security. However, Forms maintains its own "list of users" for assigning security, etc, because it has its own security considerations to worry about and really doesn't need to care about repository-level security, this latter becoming especially moot if Forms is configured to use LFDS authentication where there is no repository to base any existing security on.
For either Laserfiche repository or LFDS authentication with Forms, login access is provided to all users with either a read/write named user license or an authenticated Participant license. (Upon synchronization with the corresponding User Authentication source, that is.) If using Laserfiche repository authentication and the built-in ADMIN user has not been assigned a read/write named user license, then it is ineligible for login to Forms.
In the Forms Configuration page in the User Authentication tab, you will specify one account meeting this licensing criterion to serve as the initial "System Administrator" which you use to set up system security, etc. For LFDS authentication, there is the additional criterion that the user must belong to a whitelisted LFDS group. For Laserfiche repository authentication, there is actually the additional requirement that this user have the Manage Trustees privilege. This can be thought of as the Forms analogue of the ADMIN user in a repository.