Hi.
It's because the default scaleMode value for the stage is "showAll", which means "that the entire application be visible in the specified area without distortion while maintaining the original aspect ratio of the application".
What it means is tbat the Flash Player or the AIR runtime will resize the stage in way that it always fits proportionally inside of the window like your content was a single video file or bitmap.
You can try to experiment with other scaleMode values, but you will probably want to stick with the default value, I think.

https://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/display/StageScaleMode.html
Example of how to change the stage's scale mode:
import flash.display.StageScaleMode;
stage.scaleMode = StageScaleMode.NO_BORDER;
Regards,
JC