User blog comment:Maki Kuronami/Lesson 29: NEOTAILS/@comment-25396609-20151025155603/@comment-10380701-20151025160628

We can chalk it up to Ren growing up a bit inbetween Vol 2 and 3.

Honestly, both the Oum's lack of experience in voice acting actually helps in making their voice slightly more similar, because they don't have a distinct style of voice.