Why does my MacBook Air say that it's "up to date" with 12.2.1 when other laptops say they need 12.3.1?

Hi,

Yes, 12.3.1 is "up to date" for any Mac running Monterey. Try booting into Safe Mode: How to use safe mode on your Mac - Apple Support and then back to normal and then try Software Update again.

Not all Macs see the upgrade/update prompt at the same time. Sometimes you just have to wait a bit. Our five Macs got the prompt all the same day, but not all at the same time.