You write tests on your laptop. But they do their real job on a build server. A machine that runs your whole suite automatically every time someone changes the code. In most teams this is called CI.
Four things are different there, and each can cause a failure.
The machine is slower, and there is no screen.
Build servers are usually smaller than your laptop, and they run the browser without showing it. Any test that quietly depended on your fast machine will start failing. That is not bad luck. The server is finding timing assumptions you did not know you had made.