The Release Notes indicate what comes with the firmware update. Bug fixes might be for features and/or devices that might not apply to your specific MX or required features. So an update can be available but not really be of any use to your specific scenario, so you have the option to upgrade if you want or avoid downtime if it's not needed.
My guess would be that the change from 16.16 to 16.16.2 is only a patch and not relevant for security. So we are up to date. But when we are on the latest stable available then we see upgrade available as the next possible version is already stable release candidate.