The reflection can look composed while the person is still assembling their courage.

Outsides settle faster than insides. Someone can look entirely put together in the glass a minute before something they have been dreading, while the actual work of getting ready is still going on somewhere the mirror cannot show. This is why other people so often seem calmer than you feel, and why they are frequently not. Composure is a surface that arrives well ahead of the state it appears to report.

If you are gathering yourself before something hard, let the calm exterior be a help rather than a lie. And when somebody else looks fine, a quiet question costs little and is sometimes very welcome.