Matchmaking is broken, most people know this, you have some players that literally play like plat+ that can't get themselves out of silver or need to grind forever to do it and loads of people above that play like bronze, if this is because of account purchasing I have no idea, but I think it's safe to assume it's part of the problem.
Or at least, that's what I've heard from quite a number of people.
My brother had 1 account in high plat and one he couldn't get out of gold when he quit the game. Speaks volumes about how scewered the system is imo, so I won't bother with ranked.