The user created by Ubuntu on install is different than the ones you create using the "Users and Groups" tool - the user is "Custom" not "Administrator".
Besides being able to "sudo", what are the other differences?
Is it just groups and privileges, or is there something else?
Does install do anything else, like creating folders, files, config, that can not or are not done using the "Users and Groups" tool?
EDIT: Somewhat related question: Users and Groups Administration tool - What do the options in User Privileges tab of Advanced Settings do?