I have a theory. Maybe its stupid but ill share it anyway.
From what ive read about greys alot of then tend to resent the person that looks after them, feeds them, cleans there cage ect. My grey isnt a one person bird but is definatly nippier with me and im the one who does everything for her!
I think its like the opposite of dogs. A dog will love the person who feeds and takes care of it where as a parrot will see the person who feeds it as someone who just invades there territory every morning and evening lol!!???