changing outbound calculated differently than changing return
You hit the nail on the head: When you change the outbound Delta charges the change fee plus the difference between the fare you purchased and the lowest (unless you want to pay more) round trip fare available at the time you are making the change.
This is what all legacy carriers do. This is not what Southwest does. If you have a round trip ticket on Southwest and change only the outbound Southwest honors the purchased return fare for the part of the trip you are not changing and only reprices the part you are actually changing. I don't know about other low-cost carriers.
On legacy carriers when you change only the return, whether or not you have already flown the outbound, and if a seat is available in the fare class of your originally purchased return, then they will only charge you the change fee.
Actually now that I think about it I'm not sure if other variables enter the mix what they will do. For example if your originally purchased return fare was only good Monday through Thursday and you change to return on Friday I'm not sure what they will do. I think they will charge the change fee plus the difference for traveling on Friday using the fare that was in existence at the time you originally purchased the round trip. I think. And I think this information is embedded deep in the fare rules.