Cisco routers have different categories for authentication. A category for command line access (called EXEC). A category for PPP access. After all we wouldn't want a ISP's clients to be granted EXEC access just because they successfully authenticated their PPP session. The authentication to enter enable mode is simply one more category; a little unusual in that there is no user-supplied username.
Some authentication backends -- like RADIUS -- always need a username field. If we use those authentication backends to authenticate the enable mode category (using a configuration like
aaa authentication enable default group EXEC-RADIUS) then the router supplies the backend with
$enab15$ in the RADIUS
Note that this isn't the same as saying there is a default username for enable mode.
(The above answer assumes
aaa new-model, which is fair as it's hardly new anymore.)