index

Why does the following code place the green box on top of the other boxes in explorer, and the pink box on the top of all other browsers:
please dont just answer because explorer is *beep*.
thanks J

.grey_bg{
position:relative;
float:left;
width:100%;
height:200px;
background:#999;
}

.pink_box{
float:left;
width:100px;
background:#00ff00;
height:50px;
}

.green_overlay{
position:absolute;
top:0px;
z-index:10000;
float:left;
width:300px;
height:20px;
background:#ff00ff;
}