There is plot significance to Elluka and Ma being Luka characters. Way back before we knew that Elluka Chirclatia and Elluka Clockworker were different people, Elluka Chirclatia was viewed as being a Luka character because she would one day be the Elluka we know from Daughter of Evil in Lukana's body, and that connection is still important since Elluka Clockworker being a different person entirely is a twist (and, also, Elluka Clockworker the Luka character inhabits her body all the same). Ma, similarly, previously inhabited Lukana Octo's body and has a connection to Elluka Clockworker, to a point that for a long time viewers were led to believe that she was the same person.
Basically, it's the difference between mothy making a conscious choice to connect the characters in some way and just, using any Vocaloid for them that might happen to fit just so they can have singing lines.
For example, Anne being voiced by Rin and Mariam being voiced by MAYU. We already know that the characters are based on Sweet Ann and Miriam, but the songs featuring them used different Vocaloids presumably because Sweet Ann is an English vocaloid and Miriam isn't in available anymore, not because mothy necessarily wants to say that Mariam is a Mayu or Anne a Rin character.
For now, that's the assumption for the "non-Vocaloid" characters that mothy has voiced with Vocaloids. Presumably it's just because they wouldn't get to have lines if he didn't do that.
And we've already gone over all this before, so I'd recommend checking through the Clockwork Lullaby album thread for more thoughts on this.