Once you get into a division, you can't get dropped from it other than inactivity. All diamond players shouldn't necessarily be in diamond, but once they do get in diamond, they can't drop down. You're matched according to MMR, not division. Getting matched with people in diamond means nothing other than those people have similar MMRs and probably shouldn't have been in diamond in the first place. People are moving up and down every day using the same system you're playing on.