[@Sebastian]

I imagine different people going human may like to go down different transformation paths, ideas to cover this could be;

[list][*]Different heritages/bloodlines. 
[i]Back in the day people of the north grew massive when entering combat. Those of the south islands shone like a star. Since the great peace no one has used a transformation for 100s of generations and since the mingling of tribes very few know what powers their heritage holds.[/i]
[*]Schools of training;
[i]Having exposed your mind and body to a unique chi and rigorous training regime your transformation is set by your study. It is a unique thing that few are able to achieve but many strive for. 
While extremely rare it is possible to master two different schools and own two different transformations if one can dedicate himself to and upkeep two different schools of martial arts.[/i][/list]