[HTML5] App is off-centered in fullscreen mode


I have an almost square (760x590) HTML5 application with an option to switch to full screen by setting stage.displayState = StageDisplayState.FULL_SCREEN_INTERACTIVE. When running the app locally everything works as expected in fullscreen: the app is going from top to bottom and there are even blank bands on the sides. However, when the app is launched in iframe through a 3rd party website, it ends up being off-centered horizontally in fullscreen. It still goes from top to bottom as expected, but the blank band on the left is wider than the blank band on the right. What can be causing this issue?