Sharepoint JSOM: set_item() without update()

Question

I'm using SharePoint Javascript Object Model (JSOM). Can someone explain to me what set_item() does and what update() does?

Below is some code to illustrate.

Boilerplate code to get the list items:

var ctx = SP.ClientContext.get_current();

var list = ctx.get_web().get_lists().getByTitle('Employees');
var listItems = list.getItems("");

ctx.load(listItems, "Include(Name, Role)");
ctx.executeQueryAsync(onSuccess, onFailure);

first set of commands

var first = listItems.getItemAtIndex(0);
var second = listItems.getItemAtIndex(1);

first.set_item("Role", "foo");
second.set_item("Role", "bar");

second.update(); // only call update on the second item
ctx.executeQueryAsync(onSuccess, onFailure);

The actual network request to SharePoint contains two "SetFieldValue" methods with "foo" and "bar", and only one "Update" method for the second one. SharePoint only updated the second value with "bar".

second set of commands

first.update();
ctx.executeQueryAsync(onSuccess, onFailure);

The second request contains only a single Update method. SharePoint did not update the first value with "foo".

Question

Why is the first item sent even though update was not called?

Clearly, when first.update() is called, the new value is not passed again - but I would at least assume that it would be staged in SharePoint, waiting for the update() - but that doesn't happen.

Thanks!