Widgets aren't positioned according to padding

I created a SButton and placed it on a SOverlay.

	SVerticalBox::FSlot& playBtnLaySlot = btnLay->AddSlot();
	playBtnLaySlot.AttachWidget(playBtn.ToSharedRef());
	playBtnLaySlot.HAlign(EHorizontalAlignment::HAlign_Fill);
	playBtnLaySlot.VAlign(EVerticalAlignment::VAlign_Fill);

        // This is the important line of code
        // viewportRes contains the GameViewport Resolution
       // by using GEngine->GameViewport->GetViewportSize(viewportRes);
	playBtnLaySlot.Padding(viewportRes.X / 3.0f, 0.0f);

For some reason when I play the game the buttons are placed like this: