I have seen quite a few employers and some of them are noticeably much better at converting developer work into revenues than others. Those companies are able to offer higher salaries even to worse developers, simply because they can make more money on every quality-equivalent unit of work.
The $Y amount may be higher than $X because you are actually worth $Z to a median company, and $Y > $Z because that particular company is in the 90th percentile at turning developer work into revenues, and $Z > $X because that company is in the 20th percentile.
So, generalizing, the company that is capable of offering the most to you is going to be better at managing development teams, which is usually a symptom of being a better employer overall.
The $Y amount may be higher than $X because you are actually worth $Z to a median company, and $Y > $Z because that particular company is in the 90th percentile at turning developer work into revenues, and $Z > $X because that company is in the 20th percentile.
So, generalizing, the company that is capable of offering the most to you is going to be better at managing development teams, which is usually a symptom of being a better employer overall.