Why does the set dimensions of my figure change upon saving?

I begin my charting with the following code:

    x0 = 10;
    y0 = 10;
    width = 556;
    height = 588;
    set(gcf,'units','pixels','position',[x0,y0,width,height],'Color','k','InvertHardcopy', 'off')

I create subplot(2,1,...)

And then I save using the following code:

    saveas(gcf,'test.png')

If I save manually, there is no problem, but the code above changes the dimensions of my figure from the ones I start by setting. What is going on?