Normals/Blinds have a separate MMR from Ranked. A Challenger could be placed against Bronze players in a Normal game simply because the Challenger player doesn't play Norms a lot thus the system hasn't determined their true MMR yet. Or the Challenger might use Norms to play roles they don't do well at, so they are a Challenger Top laner, but a Silver ADC player.
If you are gold and want to be placed against other gold players, Ranked is the way to go cause the rank you are seeing is based on their current Ranked MMR. You might be a Gold 5 vs a Plat 5 depending on how good your MMR is, but you won't have Bronze and Silvers vs Diamonds in your games unless the person is intentionally sabotaging their account.
https://boards.na.leagueoflegends.com/en/c/GD/j65FKzyf-is-it-legal-for-a-bronze-player-to-have-a-diamond-mmr
[{quoted}](name=Busty Kunoichi,realm=NA,application-id=mNBeEEkI,discussion-id=j65FKzyf,comment-id=0002,timestamp=2017-02-07T15:41:45.698+0000)
Theoretically one could have challenger-level MMR by simply dodging series games in Bronze 5, never completing the series for promotion but still playing with the best of the best.
[{quoted}](name=Saminus Maximus,realm=EUW,application-id=mNBeEEkI,discussion-id=j65FKzyf,comment-id=00020000,timestamp=2017-02-07T19:04:39.320+0000)
Some madman actually did it a few months ago.
http://i.imgur.com/p4shS00.jpg
He's back in diamond 1 now, but he got to bronze 5 with challenger mmr after inting on rammus 10 games in a row to get placed low and then dodging all his promo's.