Actually, the explanation is right in the JSF Specification. The algorithm of UIComponent findComponent is used:

http://java.sun.com/javaee/javaserverfaces/1.2_MR1/docs/api/javax/faces/component/UIComponent.html#findComponent(java.lang.String)

As you see, the algorithm presume several attempts to fine the desire id in the component tree.

So, even you have:

<f:view>
 .....
 <h:graphicImage id="painter" />
 ...
 <f:subview id="foo">
 <h:form id="bar">
 ...
 <a4j:commandButton value="Redraw" reRender="painter" />
 </h:form>
 </f:subview>
<f:view>

it will still work. However, it takes extra steps before root UIComponent starts to be the base and the "painter" be found there. I.e. ":painter" helps to shortcut the way