You answered your own question in your post inadvertently. Whenever matchmaking happens, the system not only takes players' rank into account but their hidden ELO (called the Matchmaking Rating) as well--so if the Bronze1 player is on a 420-game hot streak then he may have the potential to be matched up against a Gold3 who last his lost 69 games or something.
The system is also a bit lax to cut down on queue wait-times, which would be noticeably longer (particularly at certain points of the day) if the game had to sniff out 9 players in your ballpark of relative skill. I don't agree how with the skill matchups are shuffled either though, to your point about how the team with the better botlane has the inherent advantage.