I don't doubt that the hiring company may be able to get more value from the individual, hence offering a higher salary. Market rate is highly dependent on the market, and "Software Engineer" is not a market. The current employer is in a better position to understand the employees current value to the company. They may not be tracking it, but they are in a position that they could probably understand the value of the individual, and determine if an increase in compensation is worth the current value or future value they are getting from them.