ios 18 update on the 21st

Yesterday I bought two new iPhone 16 Pro models for my wife and I. Our previous phones were 14 Pros and were updated to iOS 18. I backed them up to iCloud and restored my new phones from iCloud. Old phones running 18 were working great as are the new ones. When I woke up this morning though, I showed updates available for both phones. The updates said IOS 18. Just like it already said we were running. I did the updates and everything is still fine but what was it? Same version number. It obviously wasn’t a full update because it only took a couple of minutes to download. I’ve found no mention of it anywhere.