Okay, after having played more of this game, both online and offline, there is no denying that there is definitely something up with the items you get.
I have a theory that it might also take into account the distance between you and the one in front of you, because why else would I get a golden mushroom twice whilst driving 2nd with the number 1 having put quite some distance between me and him.
This could also explain why it's more common to get, say, a green shell or 1 mushroom whilst driving in 8th place with the number 7 right in front of you.
I mean, who gets a freaking green shell when they're in 12th place in a Mario Kart game?