This issue confused me a lot, you can make it happen as following

1. Open example from The Modern JavaScript Tutorial;
2. Open another browser window;
3. Click the new browser window. Make sure you are focus on this window and you can see both window on the same screen;
4. Try to move mouse pointer to the demo, you will get a log like below, you can see mouseenter and mouseout fired at same time

mouseout: target=DIV, relatedTarget=null mouseover: target=DIV, relatedTarget=null

By the way, im using Mac and Chrome...... Why this happen? Could you please help and explain me the reason?

Thank you very much!