One of the motivations behind the rollout of FLY is that when there are seat changes then they can be more clearly explained to the customer. I have a feeling (speculation) that you and the other couple were moved into and possibly out of a G-GAT series aircraft; but the existing seating on DCS is "plug and play" so over time it becomes very difficult to understand what happened here. In the future it should be a lot clearer. In the meantime, having experienced this on BA and indeed many other airlines - not often in all cases - I wouldn't let it get to you.