Mostly because you either didn't tell it not to or delayed an essential update too long would be my guess. Personally I always check for updates at least one a day by opening Settings then Update & Security and selecting (clicking on) the "Check for Updates" button so I never get surprised. Best time to check ( YOUR local time afternoon).
Note the taskbar "Check for Updates" only checks for Antivirus updates.
Just ignore that You're Up to Date" bit unless you have checked within the last few days at least.
THIS way you are checking for both.