Antarctica is an isolated place with unique regulations both geographically and politically.

As a result, many conflicts arise regarding the definition of "Antarctica" and therefore what counts as a race run on Antarctica. The differences in interpretations are mostly due to for-profit races attempting to "define" what is considered to be a part of the race.

Many races are held on King George Island, which is the northern-most tip of Antarctica. This is an very different type of climate than what is typically considered an "Antarctic" climate and runners would be wise to research what the locations each look like.

To save you a few googling queries, here's what each place looks like

Union Glacier

King George

While individual race directors might quarrel over this, it's worth noting that there are several reason to not consider marathons or ultra marathons run on King George Island to have considered "run on the continent of Antarctica" due to the 1) location 2) weather and 3) tectonic plate location of the island.

  1. The Shetland Islands & King George Island are located outside the Antarctica Circle [source]
  2. The weather & terrain on the outlying islands is much more temperate and not typical of the rest of Antarctica (see above photos)
  3. The tectonic plates that the Shetland islands lie on is not the Antarctic plate [source]

Wikipedia sums up the conflict like this. 

Via Wikipedia

Because of the differing opinions on what constitutes a marathon in Antarctica, there are several companies and races that lay claim to the "7 continents" name. They also all run their own independent races in Antarctica and dispute over what races actually take place on the continent.

The 7 Continents Marathon Club

This club organized by Richard Donovan explicitly excludes races taking place on outlying islands:

6. For the avoidance of doubt, the South Shetland Islands, including its northernmost King George Island, and the Falkland Islands are not considered part of the continent of Antarctica. These islands are not located within the Antarctic Circle, they are not located on Antarctica's continental landmass, and they are not part of the Antarctica Tectonic Plate or geographic continental shelf. The South Shetland Islands are specifically located on the Shetland Plate while the Falklands are on the South America Plate. Both sets of islands also share a distinctly more moderate set of climatic conditions that sets them apart from the continent of Antarctica.[8]

The Seven Continents Club

This is run by Marathon Tours and is a paid membership. They currently list 500 people that have completed the feat. This is the original "Seven Continents" club.

Official 7 Continents Marathon Club

This is run by a non-profit club, but is owned by Marathon Adventures owner, Steve Hibbs. They currently list 135 people that have finished the marathon distance, but do not qualify where in "Antarctica" their races were run.

To avoid any doubt, this site will recognize any members who have run an ultra marathon that's taken place on the continental landmass of Antarctica, that takes place on the Antarctic plate and within the Antarctic circle. 

Currently the only ultra marathon that fits all these criteria is the Ice Marathon 100k - which was recently discontinued.