The thing is, Yamato is a bit different. Luffy's will was his from the start. It happened by chance that his will was similar if not the same as Roger's.
Yamato, yes, she was inspired by Oden but she apparently emulated Oden almost obsessively. It's quite unnatural compared to the usual "inherited will" that Oda portrays. His retainers carry Oden's will but never did they ever try to emulate his mannerisms, etc. Neither did Luffy with Roger. That's because though they are different personality-wise, they still carried his will without having to bend their character to fit Oden's wishes.
I think this could be a chance for Oden to look into the psychology of Yamato. She has to realize her own self rather than trying to be Oden. So in a sense, she isn't her true self yet until she takes off the facade of Oden.