If you're referring to Cook's voyages, then the actual explanation is he didn't just circumnavigate Antarctica. He made many detours mapping various islands, backtracking, and resupplying.
[link to en.wikipedia.org (secure)]
Note that each had many detours and also included travel to and from England.

But others have circumnavigated it. There are cruises that do so and a regular sailing race. None of those take as long as it should on the mythical flat Earth map.