What is the difference between user agents & what our browsers are "announcing"?

UA vs reality

https://www.whatismybrowser.com/

Quote:

YOUR WEB BROWSER LOOKS LIKE: Opera on Linux Opera 98 on Android 12 This conflict might be causing other websites to not detect your web browser properly. It looks like you're using Android, but your User Agent is announcing that you're using Linux

How do sites seem to always be able to id our actual browser & os?