ActionScript 3.0 :: Disappoint Tracing Stage Dimension?

I want to know for what reason if i write

ActionScript Code:
addEventListener(Event.ENTER_FRAME, function()
{trace (stage.stageHeight)})

resizing the window dimesnion the trace value remain the same of the original stage dimensions setted into FLA (in my case 800px)

while writing
ActionScript Code:
stage.addEventListener(Event.RERSIZE, function()
{trace (stage.stageHeight)})

it return the REAL actual stage dimension when i resize the window.Why the first code desn't work?