Articulate Studio 13 Publishing Browser Settings and Output Slide Size

Hello heros, 

I am publishing my courses in Articulate Presenter 13 using the settings: 

Lock player at optimal size and Scale to 100% of the Powerpoint output size, the result of this is awful. It seems like the player is at a fixed size and the slide has been stretched to a bigger size and these two overlay on top of each other. I tried to use scale to 90% and this time the result is good. 

I am wondering what is the difference between choosing between 90% and 100% and why at 100% the publishing result was so off. Can someone explain to me? 

Many Thanks!