Why are (qwerty) keyboards laid out differently in the U.K. and other English speaking countries?

I’m a British expat working in South Africa, and I’m quite often called upon to use others’ keyboards. In S.A., the default layout is the ‘American’ layout.

Why is this different from the British layout? Why does the American keyboard have such a small Return key. Why is the backslash key where the return key should be. Why swap around the ” and the @ keys? Is there any reason that anyone can think of?