System update notification but system already up-to-date
Does Android put out security updates with the same version #?


Yes. Version number changes generally correspond with new features or new APIs. Occasionally it will also be incremented for major bugfixes, but this has been somewhat less common now that Google has tried to increase their release cadence (around Android 6.0-ish or so). Security updates will change the build number, but not the version number. See the list of factory images for your device for an example of this (note all the repeated version numbers, followed by build IDs). 



If you go to Settings > System > About phone you can find the "Android version" (circled blue below) and the "Build number" (circled red) of your current system. You can also check the "Android security patch level" on this screen to determine which security update you've received. They generally occur during the first week or so of each month, and they are outlined in the monthly security bulletins. Most likely you're receiving a notification for the January security update.


Picture from https://wccftech.com/android-oreo-for-verizon-pixel/