Personally I don’t think there’s a practical difference between developer and engineer, other than your proficiency.
Anyway, the cited $13k gap is interesting! My explanation would be:
Organisations which are serious/critical enough to call their developers engineers, also are the ones who care the most about choosing the right person, and also are more profitable so have more hiring budget.
They also attract the best talent, which is surely aware of their market value and have greater salary expectations.