The green check mark implies your system is secure and stable with its current updates, even when it indicates you're not up to date, usually means that your system is secure and functioning properly with the updates it currently has.
The yellow text indicates there are important updates available, but they might not be critical for immediate system security and stability.
You can explain it away, but it's not intuitive and is just bad design. Also, heaven forbid they just explain what things mean using words! But that's a different rant about form over function in current over-simplified UI's these days! Kind of like the useless "Oops! Something went wrong!" messages with no details or additional info...