Web Developers approach a website both from the inside and outside or leave some of the outsides to graphics people. Knowing programming makes a Web Developer more technical and more valuable in the marketplace.
Web Designers might be both a Developer and a Designer who can tackle the technical details and do Design. Graphic Designers have traditionally been in print design and know their craft very well but limited in their technical skills. Many Graphic Designers are migrating over to Web Designers and picking up some of the technical details but not the hardcore coding.
This is an ongoing distinction both in Colleges and in the work field. Many colleges Graphic departments now advertise their courses as Web Design but do very little of it depending on the skill sets of the educators. Their focus is still on Graphic Print Design. The colleges have two Web Design courses one being put on by the Information Technology Department and the other by the Arts or Graphic Arts department.