How does this happen?

The second leg of my trip today is DTW-ATL on 1647. I'm in paid first, and purchased the ticket in late November. There have been schedule changes, but the equipment hasn't changed, and the last schedule change was about 5 weeks ago. I'm an aisle seat "only" person and have had 3C since I bought the ticket. I do not like window seats. Early this morning, I got an e-mail from Delta saying that "due to a technical error, there has been a seat duplication" and I'm the lucky one that got moved to 3D. How the heck does this happen? And how do they decide who they're going to let keep the "original" seat? I understand that seat assignments aren't promises, but other than it being a quirk of Delta's world class IT, how can they assign the same set to two people? Did I get FAM'd? There's really nothing than I can do, except maybe gripe a little and score a few miles, but I'm really just interested in knowing how this happens.