Fantastic question Yny -
The largest contributing factor to differences in Botox treatment and results in patients of different ethnic diversity has to do with the thickness of the dermis. Different ethnicities tend to have thicker or thinner skin thickness.
Patient's with thicker are more resistant to getting the wrinkles that Botox is designed to treat in the first place however once they do get them can require larger amounts of Botox to get good correction of the wrinkles.
Typically darker pigmented skin types can have thicker dermis.
I hope this helps.