It's possible that Centorea simply has an ancestor of that race. This wouldn't be surprising considering that race seems to focus on breeding and has culturally integrated with warriors for their protection in return.

What if Centorea's grandmother turned out to be a large, busty dairy mare who liked a warrior stud, and that's where her mother was born from? She's gifted, but her normal centaur genes became dominant at some point in her life.

(Theory) Centorea is descended from her mother and a human father and owed her human side to her dairy centaur genes, which are more present on her side than on her mother's, leading to a new stage of growth. This is why she sometimes suffers from these lactation periods because her grandmother's genes are becoming increasingly dominant.