No they are not modifying your code at all. They are taking the content you provide to the public network and presenting it according to the choices of the person who wants to read your content. I believe that adverts on your pages are served from servers other than your own and I think that in general (perhaps not on your sites) people in different parts of the world will see different adverts when viewing the same page.
If I am having a slow Internet day (don't ask) I ssh into my shell account and use the links text mode browser. Am I modifying your code then? No, just using a rather odd way of viewing Web pages.