The difference appears to be that rounded rect shadows are fully opaque.
Also tested with a polygon.
shape main
{
roundrect(0,0,30,30,20,20);
rectangle(33,33,63,63);
ellipse(66,66,96,96);
startpath();
moveto(0,50);
lineto(50,100);
lineto(0,100);
lineto(0,50);
endpath();
FillAndStrokePath();
}