The title of a Necromancer holds significant weight within the entire Undead hierarchy. If we liken the Undead Mage as an ordinary living being practicing Undead Magic, then a Necromancer could be considered a formidable undead practicing magic.
Although, strict speaking, Undead Mages and Necromancers don't share the same existence, to outsiders, Necromancers are seen as the upgraded position beyond being an Undead Mage.
A plethora of reasons lead to such a situation, of which, besides the fact that many Undead Mages prefer to transform themselves into Necromancers to prolong their existence, another reason is the inherent superiority Necromancers hold over Undead Mages.
Any creature can learn Undead Magic, a type of magic that utilizes the power of the dead. Yet, it's very likely that the same magic, when cast by the living and undead respectively, the undead's magic will surely be more potent.