Why is the styling working in px but not in cm?

I have the following HTML page its basically a shift plan. But if I switch from pixels for the screen to centimeters for the printer the width do not add up anymore and thus the rows have different width.

div.line{
    white-space:nowrap;
    float:left;
}

div.a0, div.a1, div.a2, div.a3{
    border:0.1cm solid black;
    display:inline-block;
    white-space:normal;
    padding:0px;
    margin:0px;
}

div.b0, div.b1, div.b2, div.b3{
    border:1px solid black;
    display:inline-block;
    white-space:normal;
    padding:0px;
    margin:0px;
}
div.a0{width:calc(16cm - 0.2cm);}
div.a1{width:calc(8cm - 0.2cm);}
div.a2{width:calc(4cm - 0.2cm);}
div.a3{width:calc(2cm - 0.2cm);}
div.b0{width:calc(400px - 2px);}
div.b1{width:calc(200px - 2px);}
div.b2{width:calc(100px - 2px);}
div.b3{width:calc(50px - 2px);}

for(var a=0;a<2;a++){
    var b = String.fromCharCode(97 + a);
    for(var c=0;c<4;c++){
        var d = document.createElement("div");
        d.className = "list";
        for(var e=0;e<Math.pow(2,c);e++){
            var f = document.createElement("div");
            f.className = b + c;
            f.innerHTML = b + c;
            d.appendChild(f);
        }
        document.body.appendChild(d);
    }
}

JSFiddle Example