Random variables aside, the more you win, the tougher your opponent get. It's nearly impossible to get a winning (or losing) streak because of that. If you won 8 times in a row, you're going to play against someone who is most likely much better than you.
Your "average" matchup on the other hand will be calculated over a longer period of time and is determined by an invisible ladder score that pool every single players in the same big league. Your actual league has nothing to do with it, and it's very common for a high bronze to be ranked higher than a low silver.
The promotion are a bit more complex, and I'm not sure if Blizzard said how it was working, but typically, if you're fighting people from another league constantly, it often mean you will be promoted soon. Once the game decide that you win reliably enough (it can even happen after a defeat), you will be moved.
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