Why doesn't margin-right push the div element with a fixed width to the left?

Question

adding margin-right causes the div element to be pushed to the left

<div style="margin-right: 50px; background-color: blue;">
  <p>This is some content inside the div element.</p>
</div>

Why if I set the width , margin-right doesn't cause the div element to be pushed to the left?

<div style="width: 500px; margin-right: 1000px; background-color: blue;">
  <p>This is some content inside the div element.</p>
</div>