Why does ios need a black background in _transitions.scss

I noticed in the ionic CSS that within ios transitions, its setting a background color, set in the SCSS (_transitions.scss):

$ios-transition-container-bg-color:    #000 !default;

&[nav-view-direction="back"] {
  background-color: $ios-transition-container-bg-color;

Why is this, why is this not used in the android transitions? Its causing issues in our app setting the background of ion-nav-view to black, and I know I can just fix this with setting the above variable to transparent, or even the main app background colour, but my question is WHY.

Why is it used on ios and not android, it just doesnt make sense, and I dont like changing something if I dont know why its there.