There's a repeatable issue when you using Google Flights/Kayak/etc. to bring up a RT ticket on delta.com while logged in. If the outbound fare class is higher than the return, it will reprice the return to the same fare class. For example if outbound is T and return as X, it will reprice return to be T as well. Issue does not happen if outbound is a lower fare class than return. Also, the reprice does not happen if you perform the search on delta.com itself (even while logged in). It only happens when coming in via a third party search using DL's API.
Example --
https://www.google.com/flights/#sear...YTEAM;s=0;eo=e

Without more details from OP (dates/flights/etc), not much to go on here. All recent posts I have seen here (where posters have provided sufficient details) have been due to bug documented above. Also, WCCO story has been debunked (DL was pricing out broken fare return options in certain cases while not logged-in leading to a lower RT price, but reporter only looked at outbound flight options and never brought up return flights which showed that the trips were not in fact identical).