My guess is that they wanted the gathering on grades higher than your rank to be more random while making gathering on grades lower than your rank more consistent. Since skill has little to do with gathering, making luck play a larger role is the only way to control the supply of some rarer ingredients that I am sure SE will add to the higher rank grades sooner or later.

Iirc. the 2nd poll also showed that players want physical level to be made class-specific, so with that in mind I think the new system is a representation of that mindset (although the phys. level has not yet been changed).