Though Anglo-Saxon speakers share the same language, there are differences in the way they use it. Thus, American and British speakers of English show distinctions in their lexis, pronunciation and morphology when they speak their mother tongue.
A good instance of this is found in how they use different terms to refer to the same concept: lift (British English) and elevator (American English). The same happens with the pronunciation of certain words such as Lieutenant, pronounced /luːˈtenənt/ in British English and /lefˈtenənt/ in American English. Finally, the ending of some words vary from one variety to another. That’s the case, for example, of colour in British English and color in American English.
It’s clear that the message is perceived with, at least, no difference for both speakers of English. However, for any other non-native speaker the existence of these varieties makes them feel confused and, used them incorrectly, results in a wrong use of the language.
Moreover, many speakers of English firmly believe that the standard and accepted variety of English is the British one. Take a look at the following postcard from the incredible How to Be British Collection by Martyn Ford and Peter Legon published by LGP. Why is it that after several pints of beer you start speaking American English? Is that a real criticism of the way Americans use the language or just a wink at the reader, following so the satirical line of the book regarding the British culture, too?