Generally, the more dominant arm sits lower.
This is because we use our dominant arms to do more, and thus the muscles on the front of the shoulder/chest tend to be more developed. Posturally, this protracts and/or abducts the scap a bit more than the opposite side which makes it sit lower compared to the other side. In some cases it could be due to some other factors but that's the gist of it.
If you want to try to even things out make sure your non-dominant arm is as strong, and focus on shoring up any horizontal pulling strength deficit as compared to any type of pushing (bench, press, dips, etc.).