When a component is first instantiated, it usually is already invalidated or will be because you set some properties on it. The framework will then try to validate it by calling the methods mentioned in the doc. Sometimes those methods get called more than once because, for example, the validation of the display list causes a second invalidation of the measurements. Eventually, the component finishes up an updateDisplayList call without invalidating anything else, and both creationComplete and updateComplete are fired.
Once that happens, if you invalidate the component again, you will only receive updateComplete events because, the creation has been completed earlier.
