I didn't see an important point that I think is relevant for fully-remote devs and correct me if I'm wrong: On-site benefits. If you work remotely, you can't use several of the benefits a company provides so a difference in the salary is common as a compensation for those.

Same for healthcare plans and taxes. A lot of remote workers have to pay those, which in many cases are completely or partially paid by the company, so a bigger salary is needed.