The label "Username" is used in too many 3rd party menu items (and even the core one), and so the signup form labels have been corrected to match. So now, as confusing as it seems, the field: user_loginname will display as "Username" (one word) and the field: user_name will continue to display as "Display Name". The term "Login Name" should not be used at all, to avoid users being confused with choosing a "Login name" and then having to login with their "Username".
