Delta has looked into the matter, and has emailed us back to show that yes, there were only two flights rather then four.
Delta mentions that this is Expedia's "discrepancy", and that the matter must be resolved by them. This has always been true, of course. Now that Delta can see it, this will still need to be forwarded to Expedia, since they have yet to contact us about the matter. Maybe they lost our information to contact us, together with the information that's supposed to show that they were supposed to cancel the tickets over 3 months ago.
Maybe they do this to all their customers after these types of errors, and hopes that the consumer goes away.