Why do some Brits think that Churchill, Gandhi, and the Duke of Wellington are fictional characters?

I'm an American and I have to say that I was absolutely shocked when I heard recently that according to a poll, many Brits actually believe that Churchill, Gandhi, and the Duke of Wellington are characters who were made up for movies and books. Even worse, many of these same people believe that Sherlock Holmes, who is a fictional character created by Sir Arthur Conan-Doyle, was a real person! How on earth did this happen? For goodness' sake, I'm an American and even I knew that Churchill and the Duke of Wellington, who were British, were real people! Not only that, but I'm 99.9% sure that most other Americans know that Churchill was a real person, too. Can anyone explain this?