re: Ambiguities
Those things come from the time when the word was first used and the geography knowledge of that moment.
Africa is separate from Asia, no? (especially after the Suez canal was digged-Ferdinand de Lesseps, doing the same in Panama-to split North and South America)
There are continents and planets that have a number not too small but not too big either, so that it's easy to teach them to young kids (to start with).