d3.event.pageX & d3.mouse(this)[0]

I tried to figure out the difference between 'd3.event.pageX' & 'd3.mouse(this)[0]'. I guessed both are same but, when I console.log both, the value was different by '8' in my code.

var height=600;
var width=600;
var graphgap=60;

d3.csv('./details.csv').then(function(data){

var svg =d3.select('section').append('svg')
           .attr('width',600).attr('height',600)
           .on('mousemove',mousemove)

drawrect(data);

})

function drawrect(data){
    let bars=d3.select('svg').selectAll('rect').data(data);
    bars.enter().append('rect').classed('bargraph',true)
        .attr('x',function(d,i){return (i+1)*graphgap})
        .attr('y',function(d){return height-(d.Age)*5})
        .attr('width',55)
        .attr('height',function(d){return (d.Age)*(5)})
}


function mousemove(){
    let mouselocation =[];
    d3.select('svg').append('text')
    .text(d3.event.pageX)
    .attr('x',d3.event.pageX)
    .attr('y',d3.event.pageY)
    console.log(d3.event.pageX)
    console.log(d3.mouse(this)[0])
}

So, I think these two are two different things. Can anyone let me know why it makes a difference? The reason why I tried to figure this out is because I was re-writing the code below.

<script>

// set the dimensions and margins of the graph
var margin = {top: 10, right: 30, bottom: 30, left: 60},
    width = 460 - margin.left - margin.right,
    height = 400 - margin.top - margin.bottom;

// append the svg object to the body of the page
var svg = d3.select("#my_dataviz")
  .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform",
          "translate(" + margin.left + "," + margin.top + ")");

//Read the data
d3.csv("https://raw.githubusercontent.com/holtzy/D3-graph-gallery/master/DATA/data_IC.csv",function(data) {

  // Add X axis --> it is a date format
  var x = d3.scaleLinear()
    .domain([1,100])
    .range([ 0, width ]);
  svg.append("g")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x));

  // Add Y axis
  var y = d3.scaleLinear()
    .domain([0, 13])
    .range([ height, 0 ]);
  svg.append("g")
    .call(d3.axisLeft(y));

  // This allows to find the closest X index of the mouse:
  var bisect = d3.bisector(function(d) { return d.x; }).left;

  // Create the circle that travels along the curve of chart
  var focus = svg
    .append('g')
    .append('circle')
      .style("fill", "none")
      .attr("stroke", "black")
      .attr('r', 8.5)
      .style("opacity", 0)

  // Create the text that travels along the curve of chart
  var focusText = svg
    .append('g')
    .append('text')
      .style("opacity", 0)
      .attr("text-anchor", "left")
      .attr("alignment-baseline", "middle")

  // Create a rect on top of the svg area: this rectangle recovers mouse position
  svg
    .append('rect')
    .style("fill", "none")
    .style("pointer-events", "all")
    .attr('width', width)
    .attr('height', height)
    .on('mouseover', mouseover)
    .on('mousemove', mousemove)
    .on('mouseout', mouseout);

  // Add the line
  svg
    .append("path")
    .datum(data)
    .attr("fill", "none")
    .attr("stroke", "steelblue")
    .attr("stroke-width", 1.5)
    .attr("d", d3.line()
      .x(function(d) { return x(d.x) })
      .y(function(d) { return y(d.y) })
      )


  // What happens when the mouse move -> show the annotations at the right positions.
  function mouseover() {
    focus.style("opacity", 1)
    focusText.style("opacity",1)
  }

  function mousemove() {
    // recover coordinate we need
    var x0 = x.invert(d3.mouse(this)[0]);
    var i = bisect(data, x0, 1);
    selectedData = data[i]
    focus
      .attr("cx", x(selectedData.x))
      .attr("cy", y(selectedData.y))
    focusText
      .html("x:" + selectedData.x + "  -  " + "y:" + selectedData.y)
      .attr("x", x(selectedData.x)+15)
      .attr("y", y(selectedData.y))
    }
  function mouseout() {
    focus.style("opacity", 0)
    focusText.style("opacity", 0)
  }

})

</script>