Variables don't work correctly with onLoad

[panoss]

I'm working with login.fla in samples of Flash ( c:/Program Files/Macromedia/Flash 8/Samples and Tutorials/Samples/ActionScript/Login ).
I declared variable aa :
var aa = 0;

At function checkForm at "result_lv.onLoad = ...." I added:
"aa="isValidLogin = " + result_lv.isValidLogin;"

At the stage I added a textbox name aa_txt .

At submit_btn.clickHandler I added : "aa_txt.text = aa;" (after checkForm).

I login (username: Herbert password: glasses ) press button but isValidLogin does not appear at aa_txt textbox.
If I press the button for a second time, the isValidLogin APPEARS!!! at the textbox.
How can I make it appear with the first button press?
Why is this happening?

I have attached the modified login.fla (zipped).