[FreetheSouth!]

If you notice, North America is shown as an island as well. The answer is that they guessed about the parts that they couldn't check. Or, as some claim there was a warming that allowed farms and livestock to be grown there at the time of Leif Eriksson. Which might have allowed the circumnavigation of Greenland. And would account for the demise of the colony during the cooling that occurred later. Or, maybe I just et to much turkey and dreamed the whole thing.

[curmudgeonII]

No offense intended, but I prefer the turkey hypothesis.