In short: Google is apparently A/B testing yet another change to the Quick Settings layout. A user on Reddit posted today that their Quick Settings layout changed overnight. Going from three rows of three toggles, to two rows of four toggles. This gives a bit of a cleaner look, and also gives you more space to read and see notifications when the notification shade is pulled down. The user is unsure how this change happened, though some believe that it is due to the display scaling, however the user says that their display scaling is set to normal, and hasn't been changed. It's also not a case of removing a toggle or two from the Quick Settings, as you can see there is a second page of Quick Setting Toggles on this users device.
Background: Google routinely tests out different changes to different apps and even the OS itself. Typically, Google will test out these changes with a small group of users (and often times, not even inform those users that there is a test going on). So from time to time, there are people that notice differences with different apps from Google. And these are all tests that are being done. A lot of the time, these tests don't result in changes to the app or operating system itself, and that might be because of the negative feedback that Google received during the tests, or it's not something that Google wants to change. There are many reasons why.
Impact: This could be something that Google pushes out to all users in a future update, or it could be something that goes away. Either way, it does look very clean on Android 9 Pie. There are many users that would like to have this layout over the regular three by three layout that is included in Android, or even having the option to change the layout. That is also something that Google could add in the future.