I'm no expert, but I always wondered if it was a result of the language being tonal. In England, you have dialects that can sound so different that it could be another language to the untrained ear. However as English isn't tonal, there's a larger margin for error (so to speak) - accents can exist without transforming the word. Whereas with a tonal language, an accent could change the language more dramatically.
Though what's also interesting is some of the different languages and dialects in China also have different sentence structures (eg verb placements).
>However as English isn't tonal, there's a larger margin for error
True. The tones are critical in a way that can be very difficult for non-native speakers to grasp. Here are the four tones used with the words mā (mother), má (hemp), mǎ (horse) and mà (to curse) with an extra for ma indicating a question http://www.youtube.com/watch?v=QRkCf6Djprs.
And in that video she pronounces them far clearer than would be used in actual conversation.
Though what's also interesting is some of the different languages and dialects in China also have different sentence structures (eg verb placements).