Why do I now have My account and Administrator account on startup?

Administrator is a hidden account on every Windows machine, created automatically, same as the Guest account.

To check if your account has administrator privileges, you can right click on This PC and choose Manage (if you're on Windows 11, you will have to click Show more options to see the Manage button). This will open Computer Management, choose Local Users and Groups, double click the Users folder, right click your user and choose Properties, in the window that opens choose the tab Member Of - if you see Administrators written there, then your account has administrator privileges.

As to how to hide the Administrator account from the login page, https://superuser.com/questions/1365614/hide-administrator-account-from-windows-login-screen - it involves making changes in the Registry Editor.