It is because many people do not know that the island is called Great Britain. Some think the country is Great Britain, some think the empire is Great Britain, some think England is Great Britain.
Some (like me) think that the island is actually called "Britain". "Great Britain" consists of England Scotland and Wales including all their islands (so, all the ones in this quiz except for Ireland and the Isle of Man). Otherwise what is the "United Kingdom of Great Britain and Northern Ireland"? Are you trying to tell me that it doesn't include Skye, Lewis, Anglesey etc? So clearly if England, Wales and Scotland are collectively "Great Britain" then their main Island is simply Britain. Logical?
I'll give you the same analogy I just put on another quiz cardinalsin: Puerto Rico isn't a state or part of a state, but is still in the USA. The states aren't all of USA, GB and NI aren't all of the UK
I've heard that it's actually Great Britain to contrast with lesser britain (Brittany)