Edit: I remembered incorrectly, child comment has a better explanation.
Even if you win exactly 50% of your games, you will still eventually rank up. This is because you gain an extra star when you rank up. For example, let's say you start at rank 3, 4 stars.
* Win: Rank 2, 1 star
* Lose: Rank 2, 0 stars
* Lose: Rank 3, 4 stars
So even though you went 1-2, your rank didn't change.
Even if you win exactly 50% of your games, you will still eventually rank up. This is because you gain an extra star when you rank up. For example, let's say you start at rank 3, 4 stars.
* Win: Rank 2, 1 star
* Lose: Rank 2, 0 stars
* Lose: Rank 3, 4 stars
So even though you went 1-2, your rank didn't change.