First off, "spelling's" should be "spellings" above. Second, there's no difference in meaning between "grey" and "gray"; there are just two different spellings of the same word. There are a lot of words like this in English. Usually, there are different spellings because different versions of English spell it differently, i.e. there are British and American spellings of words. I don't know whether this is the case with "grey" and "gray". I'm American and I have spelled this word both ways, but I usually spell it "grey" as I think it's a more common spelling. Does that mean that's how all Americans would spell it? No, I don't know. There are words that are clearly more of an American spelling than British:
color (American) / colour (British) / eon (American) / aeon (British) /
canceled (American usually but not always) / cancelled (British) /
edema (American) / oedema (British) / anesthetic (American) /
anaesthetic (British) / apnea (American) / apnoea (British) /
capitalize (American) / capitalise (British) / practice (verb)
(American) / practise (verb) (British) / counselor (normally
American) / counsellor (British) / center (American) / centre (British).
It's just the way it is because there are so many variants and dialects of English.