I believe this is basically the "gaming the system" that @TTT103 was referring to - turning the portal locked credit into an airline eCredit. Although in your case it appears to be more organic than calculated.

Even if plans don't change pricing sure can. With the dip in Delta fares for the next few months I was able to take flights I currently booked, "change" to the exact same flights, and end up with hundreds of dollars of eCredits that I used to book an entire extra trip.

That's probably my biggest gripe with booking through Capital One / Hopper or Chase Travel these days, most of the fare rules available on that marketplace call for forfeiture of residual value. When I book with Delta directly and $800 in airfare becomes $400 on a re-price, I get multiple bites at that $400 in residual eCredit until the balance is drained to 0. If I try and do that with most portal bookings if I apply that $400 credit to a $300 flight, the remaining $100 of value just gets vaporized.

For most people I'm sure the travel credit can work out just fine; I was only looking to provide some color as to why it might not for a frequent traveler. In my mind if I'm collecting pennies playing the credit card points game I might as well collect easy dollars by re-pricing flights along the way as well. I understand that not everybody sees it the same way though.