Why dispatching mousedown works but click doesn't?

Hi,

I've experimented a strange (to me) behaviour (JavaXPCOM xulrunner
1.9.2).
On the form on tripadvisor, after typing a text in the city textfield,
eg. "Lon" the autocomplete suggestions shows up.
If I dispatch a mousedown event on the autocomplete element, then the
suggested item is correctly selected and filled in the textfield.
However, if I use the same code but dispatching a click event, is not
working. btw, the click event is NOT cancelled.

Do you have any idea why this happens? I thought that the click event
always involves  a mousedown as first event.

Thanks