Hi Laura,
I think it has something to do with the size of the users computer screen/display.
I minimized the size of the browser, then dragged the sides to either stretch or narrow the width. When I do, the image does not stay anchored to it’s location in the browser window. In other words, the width of the browser causes the image to move on the page.
Here are some snap shots showing what I mean. Each snap shot is my entire computer screen, but the browser has been narrowed or stretched to change the width, so you can see what I am talking about. See attached images.
I using a 16″ screen, so I am assuming you are using a smaller screen? Maybe 14″? Try stretching your browser screen from the sides to change the width, and I think you can probably produce the same results.
Let me know what you find out.
Thanks!