Steps to reproduce:
1. Create script with the following lines:
void OnGUI()
{
GUILayout.BeginArea(new Rect(80, 10, Screen.width - 160, Screen.height-20));
GUILayout.Button("Expect to be just one line height");
GUILayout.EndArea();
}
2. Attach the script to any gameobject in the main scene.
3. Build it for Windows Phone 8 and launch it in Emulator. Button will be taking up more than one line.

This is not reproducible on actual Windows Phone 8 device