Dropdownlist Doesn't Postback After Page_ClientValidate()?

I have just found the solution. The following function works (remove the else part):

function confirmSubmit() {
if (Page_ClientValidate("Group1")) {
return window.confirm("Are you sure to submit the form?");
}
}

But I am wondering why it doesn't work when I add the else part.

Question:

I want to have a confirm dialog after user fills in all the data in the form. I set onclientclick="return confirmSubmit()" in the submit button.

function confirmSubmit() {
if (Page_ClientValidate("Group1")) {
return window.confirm("Are you sure to submit the form?");
} else {
return false;
}
}

If Page_ClientValidate("Group1") returns false, the dropdownlist doesn't cause postback after I first select the item, and the postback only occurs when I select the dropdownlist second time.