extrudeWithRadius rounds top and bottom differently

[varalgit]

If you specify the same radius for top and bottom, the rendered shape is different for the top and the bottom. The top and bottom layers seem to be shifted by 1 layer. Example (see picture):

extrudeWithRadius(5,r1=2,r2=2,fn=10)
  offset(r = 5) 
    offset(delta = -5)  
      polygon([[0,0],[20,0],[20,20]]);


module extrudeWithRadius(length,r1=0,r2=0,fn=30){
  n1=sign(r1);n2=sign(r2);
  r1=abs(r1);r2=abs(r2);
 # translate([0,0,r1]){
    linear_extrude(length-r1-r2){
      children();
    }
  }
  for(i=[0:fn-1]){
    translate([0,0,i/fn*r1]){
      linear_extrude(r1/fn+0.01){
        offset(n1*sqrt(sq(r1)-sq(r1-i/fn*r1))-n1*r1){
          children();
        }
      }
    }
    translate([0,0,length-r2+i/fn*r2]){
      linear_extrude(r2/fn+0.01){
        offset(n2*sqrt(sq(r2)-sq(i/fn*r2))-n2*r2){
          children();
        }
      }
    }
  }

}

function sq(x)=x*x;