Setting the position of a component using setPosition positions that element relative to its containing block.
Setting the position of a component using setPagePosition positions that element relative to the entire page.
The reason you are seeing the same behaviour with both methods may be that your component is not inside any other positioned element. As a result, your component is positioned relative to the initial containing block, which is in most cases the page itself.
You can read more about positioning and containing blocks in the CSS spec section on Positioning Schemes.
but wouldn't 'relative to the page position' require absolute positioning? both calls assign 'position: relative' to the element.
i see that setPosition calls x.getElement().makePositionable(), and if i call makePositionable() myself, the element will have an absolute position, but setPostition does not end up with an absolute position...