bbBox vs. getBoundingClientRect

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>
<div id="headerDiv">	
  <h1>Mouse Coords: <span id="mouse" style="color:red"></span></h1>
  <h1>bbBox Coords: <span id="bbBox" style="color:red"></span></h1>
  <h1>getBoundingClientRect Coords: <span id="clientRect" style="color:red"></span></h1>
</div>
<body>
  <script>
    
    
    d3.select("body").on("mousemove",function(){
      	console.log(d3.event)
        d3.select("#mouse").text("x: " + d3.event.clientX + ", y:" + d3.event.clientY)
      
    })
    
    var svg = d3.select("body").append("svg")
      .attr("width", 960)
      .attr("height", 500)
    	.style("border","1px solid black")

var circle = svg.append("circle")
      .attr("cy", 200)
      .attr("cx", 120)
	    .attr("r",10)
      .attr("font-size", 36)
      .attr("font-family", "monospace")
		
   d3.select("#bbBox").text("x: " + circle.node().getBBox().x + ", y:" + circle.node().getBBox().y)
   
   d3.select("#clientRect").text("x: " + circle.node().getBoundingClientRect().x + ", y:" + circle.node().getBoundingClientRect().y)
   
   console.log(circle.node().getBBox())
   
  </script>
</body>

https://d3js.org/d3.v4.min.js