This is because you are operating on an element collection.
getChildren (and lots of others) return a collection ('Elements' in
MooTools) which is an array-like-object containing the elements. If you
execute an Element method on it (setStyle, set.. or getPosition) it will
call the methods on all elements in the array. If those methods have return
values other than the element itself, they will be in-place instead of the
element.
Example:
var elements = element.getChildren();
// [p, span]
elements.set('text', 'test');
// returns again, [p, span]
elements.getStyle('color');
// returns an array [#fff, #000] for example.
