Status is not up-to-date
Some people are online but status shows Away; some people are actually offline but status shows Online; some people are online but if they don't actually open Teams window, status shows they are offline (means if they minimize, status shows offline). These kinds of situations happen a lot during our use.
I noticed this too: when I work in another application (on a secondary screen) for a while, my status changes to Offline in Teams. This shouldn't happen