Easy. (1) mistaken calculation, and (2) if he followed the coastline, coastlines are convoluted and can be extremely long. Nobody endorses Cook's number today. Magellan traveled more than 24,000 miles because he did not follow a straight course (had to sail around the tip of South America, for example).
Captain Cook went around Antarctica and estimated it was over 60,000 miles explain that on a ball with a circumference of only 24,000.
Easy. (1) mistaken calculation, and (2) if he followed the coastline, coastlines are convoluted and can be extremely long. Nobody endorses Cook's number today. Magellan traveled more than 24,000 miles because he did not follow a straight course (had to sail around the tip of South America, for example).