September security update / full Oreo install

I sideloaded the 8.0 update just as soon as the OTA was posted, maybe a week and a half ago now. The "security patch level" was listed as August 5. Tonight I checked updates to see if the September security update was available yet. I received a notification to update to 8.0 with a full 994 megabyte download (or something like that, just under 1 gig). When the update finished, I looked in system info and noticed that it's now showing security patch level as September 5. Why the full reinstall of Oreo? Has anyone else had a similar experience? FWIW I'm using N6p on Verizon.